Use trailing undescore to disambiguate entity names from python keywords

[pvandyken]

Currently, the from entity, not yet officially part of BIDS spec afaik but still found withing the derivatives config in pybids, overlaps with the python keyword from, as in from bids import BIDSLayout. As a result, the following cannot be run without a syntax error:

layout.get(from="space", to="other_space")

The convention for such cases is to add a trailing underscore to the reserved word (see PEP8), e.g. from_. I propose we allow the use of such underscored arguments in the .get method (and wherever else it's relevant).

This could be on a entity-by-entity basis, but as underscores in entity names are not valid or sensible in BIDS anyway, I think it would safe enough to just remove trailing underscores from every filter passed to bids. This would cover any future cases (e.g if class was ever made an entity).

[pvandyken]

It is possible to work around this using:

layout.get(**{"from": "space"})

but it's a super awkward syntax.

[pvandyken]

If you're interested, I'm definitely able to implement this, but I likely won't have time in the near future

[effigies]

I think I would be okay with removing trailing underscores from filters, unconditionally. @adelavega?

[adelavega]

Can't think of why not