[Tentative] Add tests/assertions for allowed x_specs fields, implement x_specs['user']

[jlnav]

Related to #266

'x' refers to either 'sim', 'gen', or 'alloc'.

Considering what data is necessarily passed to user functions, how to best differentiate between libE-reserved fields and user-defined-fields, and how to support sim_f/gen_f's with differing output data-types, we seem to now be leaning towards option 2 from #266 .

This will require several updates:

• Implement checks and assertions that libE/user fields are defined in their proper structures. For example, keep user-defined sim fields in sim_specs['user']

• Construct tests for these assertions, also update formatting for other tests.

• Update documentation, including examples and tutorials

Feel free to update this issue and/or offer suggestions for proceeding.

[jmlarson1]

@jlnav I think this is somewhat addressed by PR #271, except the update of documentation. I'm not sure what other assertions we might need, other than checking that x_spec keys match a pre-defined list. (After compiling that list in libE.py, I think we should consider moving some things to libE_specs, but that is later.)

[jlnav]

Presumably closed by #271