add a test suite to the repo via pytest, and fix some minor bugs in _minute_gridder.py uncovered by the test rewrite

Mambaforge already has mamba installed, so this was unnecessary. In addition, it was causing builds to fail due to an incompatibility between mamba & conda versions. Related ticket: https://github.com/mamba-org/mamba/issues/1706

new satpy version to fix #27 - error when gridded file prefix is > 2 characters and tiles are generated

update satpy hash to post-https://github.com/pytroll/satpy/pull/1548, with support for modifying the environment prefix

write a conda environment.yml to the python_runtime/ folder so that we can more easily audit what's included in our runtime between builds

add compression to the tiles produced by satpy. also gets us other satpy updates like the static scaling factors. (fixes #14)

migrate the pip dependencies from package.sh to our conda environment file. this will include them in our buildbucket & keeps our package script leaner.

adds a .gitlab-ci.yml which builds the package and uploads artifacts. we also now use the master branch from glmtools.

Requires djhoese github branch for glmtools to work without matplotlib