add support to run python functions in environments

[satra]

right now the environment classes only support shell commands. however, in general our workers support python functions to be executed from jobs.

perhaps we can use that concept and refine it to move function pickles between environments. this does mean that environments should have necessary components for execution, but i think that's the kind of thing that we were looking for in senselab. i haven't seen a clear python analog for this, but it would be like executing a python script instead of a shell command. since we cache the input object anyway, i can see a potential for writing out a script and a dependency list to run it.

this would require a bit of changes, but may open up a lot of possibilities to combine workflows that include different versions of python libraries and more minimal environments with less friction.