set up CI

[germasch]

• should test with/without cuda

• should test with/without ADIOS2

• should run unit tests

• should run small test case for a couple of steps

• should run small test case with checkpoint/restart

• should run small test that actually rebalances

[germasch]

Some of this has actually happened, but I guess there's always more, so the reminder can't hurt...

[JamesMcClung]

Alas, the CI does not currently test with cuda...

The workflows that ostensibly tested cuda were removed in #387 ; it'd be good to add a workflow back (and make sure it actually works).