Isolating optional dependencies

Policy

Optional dependencies should not be included in Firedrake's global namespace and, if the dependency is wanted by most users and hence installed by default (e.g. matplotlib), firedrake-install should include a flag to skip installing it.

Details

By "global namespace" we are referring to the classes and functions imported with the command from firedrake import *. Optional dependencies should be kept in their own modules/subpackages and only imported with an extra import command (e.g. from firedrake.ml import *). Running these import commands without the appropriate packages installed should raise an ImportError with an error message that explains how to obtain the necessary packages.

Motivation

Optional dependencies can often be a headache to install (e.g. pytorch, matplotlib, VTK) and users should not be forced to deal with complicated installation issues for packages that they have no intention of using.