Resolve "Random error in `test_gvec_equil.py`"

Solves the following issue(s):

• Closes #360 (closed) and #361 (closed)
• Remove pytest-monitor package and its use in console. This gets rid of the annoying pymon error when locally running parallel unit test. We were not using the pytest-monitor timings anyway, so we can remove unnecessary code.

Core changes:

• struphy test unit now executes two tests:
  • single-process unit tests (@pytest.mark.mpi_skip), which now include the gvec test among others (which were not tested before)
  • the usual 2-process tests (@pytest.mark.mpi(min_size=2)).
  • If a unit test has neither of the two specifications it will be executed in both instances.
• clean-up some struphy console functions; in particular added the function subp_run which launches a subprocess and prints the command on screen.
• Added -v (--verbose) flag to struphy run command and to StruphyModel base class; by default the major outputs of the model initialization are now suppressed (see the model tests for instance).

Model-specific changes:

None

Documentation changes:

None