Do not import PETSc at the top-level

[connorjward]

This avoids us initialising PETSc before petsctools.init is called.

Note that this does result in a small API change. We now have OptionsManager.get_commandline_options() instead of OptionsManager.commandline_options. I don't know a better way to do this.

[JHopeCollins]

1. Could you put an if-guard and a warning around the import to avoid the API change?

2. Would something like the following work to avoid the API change:

class OptionsManager
    @classmethod
    def _get_commandline_options(cls):
        from petsc4py import PETSc
        return frozenset(PETSc.Options().getAll())

    @property  # or @cachedproperty
    def commandline_options(self):
        return self._get_commandline_options()

[connorjward]

1. Could you put an if-guard and a warning around the import to avoid the API change?
2. Would something like the following work to avoid the API change:

class OptionsManager
    @classmethod
    def _get_commandline_options(cls):
        from petsc4py import PETSc
        return frozenset(PETSc.Options().getAll())

    @property  # or @cachedproperty
    def commandline_options(self):
        return self._get_commandline_options()

@property does not seem to work with class methods unfortunately. I tried something like this.

[JHopeCollins]

Seems to DTRT again now for command line arguments. Happy for this to be merged once the linting is passing.