Issue 3656: Implementation of fancy indexing in matrix
give Sparse an extract method; let Sparse use fancy indices
remove testing for slice in a2idx
edit submatrix doc and copy docs into sparse.py
Corrected cross product determinant representation.
Merge pull request #2641 from pestrickland/master

Mechanics documentation correction
Add Paul Strickland to the AUTHORS/aboutus. Welcome to SymPy!
watch for real and imag parts being equal in solve
Merge pull request #2643 from smichr/gh2642

watch for real and imag parts being equal in solve
Merge pull request #2622 from smichr/3565

Issue 3656: Implementation of fancy indexing in matrix