Add requirements file

[tskisner]

Hosting sotodlib on PyPI is a work in progress, pending so3g/spt3g work. However, when pip installing sotodlib we could already provide a requirements.txt file that eases the installation / verification that dependencies are present.

[kmharrington]

Could we revisit this? We need a requirements file for the docker install of sotodlib that goes inside of the sodetlib docker image.

A couple notes:

• @jlashner Does the docker image building automatically look for requirements files and install those packages?
• Is toast ever going to be a requirement for sotodlib? I believe we decided on no but want to make sure we're still good with that. Because we definitely don't want to require toast on the smurf servers.

[kmharrington]

In particular this error shows up because our filtering functions need pyfftw but that's not installed in the sodetilb docker. But I think adding / using the requirements file is actually the correct way to fix that problem

[kmharrington]

Does this part of setup_opts effectively work as a requirements file? https://github.com/simonsobs/sotodlib/blob/master/setup.py#L49

[tskisner]

The long story about requirements files and setup.py is here:
https://packaging.python.org/en/latest/discussions/install-requires-vs-requirements/#requirements-files
https://caremad.io/posts/2013/07/setup-vs-requirement/

But for our purposes (assuming all requirements are listed in setup.py) we could just add a requirements.txt file that had:

--index-url https://pypi.python.org/simple/

# By default, use our abstract dependencies in setup.py for the
# concrete dependencies from PyPI.
.