Improve radical_basis and cartan_invariants_matrix for a finite dimensional algebra

[nthiery]

This ticket improves the algorithmic complexity (n^4 to n^3 for radical_basis) and further optimizes the code for computing the radical and the Cartan invariants matrix.

Without:

sage: A = HeckeMonoid(SymmetricGroup(5)).algebra(QQ)
sage: %time len(A.radical_basis())
CPU times: user 4.25 s, sys: 45.1 ms, total: 4.3 s
Wall time: 4.26 s
104
sage: %time A.cartan_invariants_matrix()
CPU times: user 45.2 s, sys: 267 ms, total: 45.4 s
Wall time: 45.5 s

With:

sage: A = HeckeMonoid(SymmetricGroup(5)).algebra(QQ)
sage: %time len(A.radical_basis())
CPU times: user 418 ms, sys: 29.5 ms, total: 447 ms
Wall time: 422 ms
104
sage: %time A.cartan_invariants_matrix_by_characters()
CPU times: user 9.39 s, sys: 208 ms, total: 9.6 s
Wall time: 9.53 s

(the above examples do not use that this is a monoid algebra, though of course the sparsity helps).

The dependency on #6484 is only for speed.

[nthiery]

Last 10 new commits:

​cba7c8d	17696: factored out of the examples a basic implementation of the 0-Hecke monoid
​6e81e4b	16659: proofreading and little additions to the doc; small refactoring of the code
​19756de	16659: minor line-split in the doc
​fd8e1ad	16659: refactored _orthogonal_decomposition and updated doctests accordingly
​cc327ef	16659: improved documentation for _orthogonal_decomposition
​e00dfdb	16659: improved documentation for _orthogonal_decomposition
​cce7d3d	17696: added crosslinks
​faf83a1	16659: added missing hecke_monoid.py file
​223e558	18310: Finite dimensional modules with basis: improved conversions to vectors and matrices
​50867d8	18311: O(n^3) algorithm for radical_basis in char 0 and faster Cartan invariants matrix by characters