add a sparse_view class that wraps pre-allocated data into a matrix expression

[Gunter]

Provide a way to use ublas with pre-allocated data. Implement a matrix view of a CRS matrix as proof of concept. Details:

• given 3 arrays: row pointers, column indices, values (​http://www.netlib.org/utk/papers/templates/node90.html)

• provide a wrapper that fulfills the immutable part of the matrix expression

Further tasks:

• split the current matrix and vector concepts into immutable and mutable parts in order to gain a "read-only view" concept and a full featured "expression" concept

• improve traits mechanism and apply it where possible to automatically see a fixed size C-array as vector view or matrix view

• add necessary tests

[Gunter]

(In [56003]) see #3396

traits.hpp:

• split traits into "view traits" and "mutable traits"

experimental:

• added new folder for emerging/unfinished features

experimental/sparse_view.hpp:

• added new class c_array_view which presents a T* array and a size as vector_view

• added new class compressed_matrix_view which wraps three arrays into a matrix_view

detail/concepts.hpp:

• prepared vector_view concept (still disabled because further work is needed here)

[Gunter]

(In [56005]) see #3396

test/concepts.cpp:

• prepared test for vector_view and sparse_view (still disabled)

test/sparse_view_test.cpp:

• test case for sparse_view, currently only operator(i,j) is tested

test/Jamfile.v2:

• added new test case to test suite

[Gunter]

(In [56006]) see #3396

doc/expression_concept.htm:

• splitted required methods into to const and non-const methods
• fixed minor typos

[Gunter]

rescheduled in favor of other tasks

[Gunter]

(In [56624]) see #3396 (view concepts), see #3449 (new free functions)

• merged [51765],[56003],[56600],[56602],[56621] into release
• added new free functions to simplify learning uBLAS