[yt-dev] Issue #744: Standardize geometry selection (yt_analysis/yt)

[Matthew Turk]

New issue 744: Standardize geometry selection
https://bitbucket.org/yt_analysis/yt/issue/744/standardize-geometry-selection

Matthew Turk:

Currently, there is some ambiguity in how the lengths are passed in in the selection routines.  For the most part, everything is (currently) done correctly in "code_length" units, but this needs to be made explicit.

"code_length" itself is no longer mandated to be a part of the lookup registry.  This was to prevent errors.  I think that we may eventually want to mandate everything in "unitary", but for now I will attempt to implement in "code_length" in the Cython code.

To complete this, essentially every place in `yt/geometry/selection_routines.pyx` that a geometric ndarray is accessed will need to be converted to "code_length".  I am inclined to think that we can use in-place conversions here, to avoid making duplicates of the arrays (which may be large).

Responsible: MatthewTurk