Change CI to be OS indepedent

[pedrobcst]

As of possible new release (1.1b), we might support all OSes. In light of this, it might be necessary to update the CI to test in all oses.

This (hypothetically) might work [basically move to conda for enviroment management for CI]:

• [ ] Create enviroment file for conda that install all Xerus dependencies
• [ ] Create new enviroment using this file, so there is no need to do apt-get to install gfortran etc (it will come with conda)
• [ ] Set the tests to run on windows-latests / macos-latest