github actions instead of travis ci

[henrikef]

This implements github actions workflow as already implemented for threeML.

Tests run, but still fail due to known issue #45

Also added check for stale issues and issue templates (bug report, feature request, analysis question)

[grburgess]

Hmm. I cannot see the actions actually running myself.
It looks cool. The only question I have is if it is a good idea to pull the master on 3ML rather than from pip?

I think it its smart to pull the master because this way things will stay in concert between repos if updates are made in both. However, that would mean that someone installing from pip only one of the repos might get different results. Opinon(s)?

[henrikef]

Hm. Right now, I'm installing astromodels & threeML from conda in the tests, which is consistent with the recommended installation procedure for hawc_hal. I guess I could add another set of tests using the current master branch of threeML/astromodels instead.

[grburgess]

@henrikef ah... somehow my brain saw that you were installing from the master at GitHub... sorry for the confusion. Must sleep