allow parsing function to add_py_function

[martinvonk]

This PR allows for parsing a function/callable to the PstFrom.add_py_function method.

Originally, only a file name is allowed. This (separate) file was read, and the corresponding function in the file was added to the PstFrom class. Now you can directly add a function, available in the same script as the one where the PstFrom class is initialized.

Solves #527

[briochh]

Hi @martinvonk, this looks like a nice touch. Do you think you could add to or amend a test to run this option? There should already be one in pst_from_tests that is hackily doing it the old way with a function in the current file.

[coveralls]

coverage: 79.631% (+0.07%) from 79.565%
when pulling 8a52226 on martinvonk:py_function_callable
into 459eda8 on pypest:develop.

[martinvonk]

There are multiple test where the current method with __file__ is tested. So I took one line in the tests with:
pf.add_py_function(__file__, '_gen_dummy_obs_file()')
and replaced it with:
pf.add_py_function(_gen_dummy_obs_file, '_gen_dummy_obs_file()')

Hopefully that is what you wanted me to do :)

[briochh]

Thank @martinvonk, good addition!