Clean multipatch domain

[yguclu]

Require orientation of interfaces (ornt parameter) between patches in a multipatch domain when:

• Creating an Interface object (in sympde.topology.basic)
• Creating a Domain object (in sympde.topology.domain) using either join or from_file constructors

Moreover:

• Make Connectivity.todict() store the orientation in the output dictionary, hence allowing Domain.export() to save that information to file
• Add consistency checks when creating a MappedDomain object (from sympde.topology.mapping)
• Clean up module sympde.expr.evaluation and expand its docstrings
• Clean up unit tests in modules:
  • sympde.expr.tests.test_expr
  • sympde.topology.tests.test_logical_expr
  • sympde.topology.tests.test_topology
• [CI] Use pytest-xdist to run unit tests in parallel (excluding sympde.topology.tests.test_gallery, because h5py is not thread-safe)
• Update library version to 0.20.0

[campospinto]

Thanks, that's a lot of cleaning!

I don't know if the PR should be merged as is, I just left a few comments for now

[campospinto]

These changes are useful and I think can be merged. Thanks!