Modifications to snIa.py
- Checked that I can pass multiple bandpass names in ObsMetaData
- Added a attribute of the class called survey offset that adds an offset to t0 samples. Hard coded. Reported in https://github.com/rbiswas4/SNIacatalogs/issues/1
- Used the cannot_be_null feature of instance catalogs to create a window of size SN_thresh defined in the instance catalog, outside of which no SNIa are taken into consideration. The window is hard coded to 100 days, reported in https://github.com/rbiswas4/SNIacatalogs/issues/1

Added plots.py :
New file, hopefully start to monitoring properties of catalog objects
Seds.ipynb uses SNIaCat.txt to load an SNCosmo Model and do magnitude calculations with LSST filters comparing them to SNcosmo results.
- Seds.ipynb explores the transformations in sed files.
- Tries to match the mags obtained from LSST and sncosmo, though this does not seem to work correctly.
- I also got a failure to calculate the iband flux/mag from sncosmo (obtaining nan instead). I checked for band_overlap and that was not the problem. So this needs looking into.

snIa.py:
- no real changes ... have to get get_mags working.