Incorporating CMB Lensing Sources and Statistics into Firecrown for 5…

…x2pt (LSSTDESC#123)

* read CMBK

* adding and testing CCL NK and GK

* adding NK & GK

* Update sources.py

* add documentation

* fix lint errors

* few fixes

* Update sources.py

* Update sources.py

* Update sources.py

* Update sources.py

* Add CMBL test

* Update test_cmbl_source.py

* Update test_cmbl_source.py

DOC add a bit (LSSTDESC#117)

ugh why

ENH allow all kinds of covariances (LSSTDESC#104)

* ENH allow all kinds of covariances

* TST remove outdated tests

changes again