ENH: make apply_inverse realtime friendly
FIX: Use bool param to say if inv is already prepared
ENH: Add option of prepared inv operator to other apply_inverse functions
TESTS for prepared param + speedup
DOC: make_inverse_operator also generates inverse operator
DOC update whats new + fix test
Merge pull request #1496 from mainakjas/speedup_inv

ENH: make apply_inverse realtime friendly + faster in loops