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
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