Introducing GitHub Actions CI

[ferdonline]

Description

This PR introduces GitHub actions-based CI

The strategy is to test for several Linux (Ubuntu) systems, for several compilers, Mac and Windows
Total: 10 jobs

Combinations

Linux_MPI:
    os: [ubuntu-16.04, ubuntu-18.04, ubuntu-20.04]
    compiler: (default/old)
    options: PARALLEL_HDF5 + XTENSOR + EIGEN

Linux_Compilers:
    runs-on: ubuntu-18.04
    compilers: [clang-8, clang-9, gcc-8, gcc-9,  gcc-10]
    options: EIGEN

MacOS: (default compiler + EIGEN)

Windows: (Default compiler + EIGEN + Xtensor)

Fixes #402

[alexsavulescu]

Suggested change

	- ci_test
	- master

[ferdonline]

I think it is a good idea to leave master out, since push to master is only allowed with PRs, which are already tested.
having a special ci_test is debatable, the idea is to have a special branch so that we run CI every time we push, which is useful mostly to develop the CI itself. Do you suggest another way? @alexsavulescu

[alexsavulescu]

I'd say it's standard practice to run CI against master. It's true it would virtually be redundant wrt PRs, regardless of merge strategy. Additional steps can be done if isMaster() like storing artifacts, publishing documentation, displaying CI badges and so on. I guess we can look at it also as a sanity check.

having a special ci_test is debatable, the idea is to have a special branch so that we run CI every time we push, which is useful mostly to develop the CI itself. Do you suggest another way? @alexsavulescu

The CI will run for each PR every time a new commit is pushed even when working on the CI, so there is no need for a special branch. To work on the CI itself, the most straightforward (and isolated way) is to use a fork, where you can modify master directly or even use PRs (if you don't want to add master branch on push).

[alexsavulescu]

LGTM