Sncosmo

- Add core-collapse models from Vincenzi et al. (2019)
- New Spectrum class to handle processing and fitting spectral observations.
- Removed appveyor builds.
- Bugfixes:
- Prevent segfaults in light curve fitting when the minuit fit fails.

- Add version 2.0 of many `snana-...` built-in core-collapse models, based on Pierel et al. 2018 (229).

- Bugfixes:

- Fix compatibility with scipy 1.3+ by removing outdated import statements (238).

- Fix issue affecting optimization of models with free propagation effects (236).

API Changes
- Registry functions moved to the top-level namespace:
- `sncosmo.registry.register()` -> `sncosmo.register()`
- `sncosmo.registry.register_loader()` -> `sncosmo.register_loader()`
- `sncosmo.registry.retrieve()` -> deprecated, use class-specific functions such as `sncosmo.get_bandpass()`.

The old import paths will still work, so this is backwards compatible.

Enhancements
- `nest_lc()` now uses the `nestle` module under the hood. A new
keyword `method` is available which selects different sampling
methods implemented by `nestle`. The new methods provide potential
efficiency gains.
- The MLCS2k2 model is now available as a built-in Source, with the
name `'mlcs2k2'`.
- Bandpasses from the Carnegie Supernova Project added to built-ins.
- In `realize_lcs()`, a new `scatter` keyword makes adding noise
optional.

In addition, there have been several minor bug fixes and
documentation improvements.