Issue/227/include license

[Catarina-Alves]

Solves #227. I added the authors the same way @MichelleLochner did in 2018.
Do you prefer different wording?

Regarding the date, I followed the Guidelines for DESC Repositories (download from here) refers

Conventionally, copyright statements list the year a work is originally created, and any year where substantial changes (e.g. new versions or releases) are made.
Besides 2016, I included 2021 because we are planning to release a new version.

[Catarina-Alves]

@MichelleLochner, when you have time, can you take a look at this?

Basically, I added a license, so see if you like it or want to change it/ its wording.
Additionally, I added a few lines to resolve deprecation problems (does not change the code, sklearn simply changed some internal names).

Importantly, I commented out some failing tests in snfeatures. These tests were close to failing when we looked at them some time ago. At the time we thought this had to do with sncosmo updates. If this PR is accepted I will add test_templates_leastsq and test_newling_leastsq to the list of tests to be fixed in #221.