ADD: new track message, Entity class and Position class

libs/eigen/doc/TutorialMapClass.dox

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+namespace Eigen {
+
+/** \eigenManualPage TutorialMapClass Interfacing with raw buffers: the Map class
+
+This page explains how to work with "raw" C/C++ arrays.
+This can be useful in a variety of contexts, particularly when "importing" vectors and matrices from other libraries into %Eigen.
+
+\eigenAutoToc
+
+\section TutorialMapIntroduction Introduction
+
+Occasionally you may have a pre-defined array of numbers that you want to use within %Eigen as a vector or matrix. While one option is to make a copy of the data, most commonly you probably want to re-use this memory as an %Eigen type. Fortunately, this is very easy with the Map class.
+
+\section TutorialMapTypes Map types and declaring Map variables
+
+A Map object has a type defined by its %Eigen equivalent:
+\code
+Map<Matrix<typename Scalar, int RowsAtCompileTime, int ColsAtCompileTime> >
+\endcode
+Note that, in this default case, a Map requires just a single template parameter.  
+
+To construct a Map variable, you need two other pieces of information: a pointer to the region of memory defining the array of coefficients, and the desired shape of the matrix or vector.  For example, to define a matrix of \c float with sizes determined at compile time, you might do the following:
+\code
+Map<MatrixXf> mf(pf,rows,columns);
+\endcode
+where \c pf is a \c float \c * pointing to the array of memory.  A fixed-size read-only vector of integers might be declared as
+\code
+Map<const Vector4i> mi(pi);
+\endcode
+where \c pi is an \c int \c *. In this case the size does not have to be passed to the constructor, because it is already specified by the Matrix/Array type.
+
+Note that Map does not have a default constructor; you \em must pass a pointer to initialize the object. However, you can work around this requirement (see \ref TutorialMapPlacementNew).
+
+Map is flexible enough to accommodate a variety of different data representations.  There are two other (optional) template parameters:
+\code
+Map<typename MatrixType,
+    int MapOptions,
+    typename StrideType>
+\endcode
+\li \c MapOptions specifies whether the pointer is \c #Aligned, or \c #Unaligned.  The default is \c #Unaligned.
+\li \c StrideType allows you to specify a custom layout for the memory array, using the Stride class.  One example would be to specify that the data array is organized in row-major format:
+<table class="example">
+<tr><th>Example:</th><th>Output:</th></tr>
+<tr>
+<td>\include Tutorial_Map_rowmajor.cpp </td>
+<td>\verbinclude Tutorial_Map_rowmajor.out </td>
+</table>
+However, Stride is even more flexible than this; for details, see the documentation for the Map and Stride classes.
+
+\section TutorialMapUsing Using Map variables
+
+You can use a Map object just like any other %Eigen type:
+<table class="example">
+<tr><th>Example:</th><th>Output:</th></tr>
+<tr>
+<td>\include Tutorial_Map_using.cpp </td>
+<td>\verbinclude Tutorial_Map_using.out </td>
+</table>
+
+All %Eigen functions are written to accept Map objects just like other %Eigen types. However, when writing your own functions taking %Eigen types, this does \em not happen automatically: a Map type is not identical to its Dense equivalent.  See \ref TopicFunctionTakingEigenTypes for details.
+
+\section TutorialMapPlacementNew Changing the mapped array
+
+It is possible to change the array of a Map object after declaration, using the C++ "placement new" syntax:
+<table class="example">
+<tr><th>Example:</th><th>Output:</th></tr>
+<tr>
+<td>\include Map_placement_new.cpp </td>
+<td>\verbinclude Map_placement_new.out </td>
+</table>
+Despite appearances, this does not invoke the memory allocator, because the syntax specifies the location for storing the result.
+
+This syntax makes it possible to declare a Map object without first knowing the mapped array's location in memory:
+\code
+Map<Matrix3f> A(NULL);  // don't try to use this matrix yet!
+VectorXf b(n_matrices);
+for (int i = 0; i < n_matrices; i++)
+{
+  new (&A) Map<Matrix3f>(get_matrix_pointer(i));
+  b(i) = A.trace();
+}
+\endcode
+
+*/
+
+}