Enabling support for process decompositions with empty pencils.

[romerojosh]

Fixes #101.

As it stands right now, cuDecomp will not allow users to create grid descriptors that potentially result in situations where processes have empty pencils with zero elements in any pencil orientation (i.e. x-pencils, y-pencils, z-pencils), which is very conservative. Users might desire using cuDecomp on only a subset of the pencil orientations and might not care if the other orientations have empty processes. For example, in #101, that user is interested in using halo-exchange routines on z-pencils with N x N x 1 dimensions. The current empty pencil checks will not allow this usage at all due to the x- and y- pencil orientations attempting to distribute the 1 dimension.

This PR addresses this limitation by:

1. Loosening the empty pencil checks during halo autotuning to only the pencil orientation being tested.
2. Removing the general restriction on creating grid descriptors resulting in empty pencils.
3. Allowing cuDecomp to work on distributions resulting in empty pencils where possible:
   Transpose APIs will work in these scenarios as expected.
   Halo exchange APIs will throw an error if the halo exchange is along a dimension with an empty pencil as what the halo exchange operation should do in this configuration isn't very well defined.

An alternative option to (3) is to just have cuDecomp throw an error whenever it detects communication involving dimensions with empty pencils. This would still allow use cases like #101, but with reduced complexity introduced from the introduction of empty pencil handling. This may be the preferred solution but not sure at the moment.

[romerojosh]

Going to close this on in favor of #103.