start adding code for flat-sky map simulations.
update documentation for cinv/cd_monitors.py
add cinv/opfilt_teb.py, a module containing C^{-1} filtering code for temperature+polarization maps.
add cinv/multigrid.py as a possible C^{-1} preconditioner.
add a test script (examples/cinv/test_cinv_teb.py) for sanity-checking the cinv module.