Update and get basic tests runnning

[Tokazama]

• is_time -> is_temporal: b/c the package is called SpatioTemporalTraits
• Removed direct reference to axes and keys for public functions. This comment worked on me for a while. The goal is to have sensible defaults that may be overloaded elsewhere. So it won't always make sense to have spatial_keys, but spatial_indices will make sense whether working with an array, plot, or mesh.
• All things should be inferable now.
• Given the conversation in Interop/testing #2, onset is now time_first and time_end was changed to time_last for symmetry. Also spatial_offset is now spatial_first. The short of the conversation is that the onset and offset are terms that may be interpreted differently depending on whether the first time point/spatial index coincide with the exact location of corresponding values or are off center from it as a reference.
• A very minimal set of metadata options are thrown in there right now. Mostly as a starting point and not a final design decision.

[codecov]

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