Add NeXpy plugins directory

This is to be installed in ~/.nexpy/plugins for a top-level menu to be
added to NeXpy.
Makes nxmerge more generic

Now it doesn’t initialize the omega array, since that cannot always be
defined at the merge stage.
Adds a path to the parameter list

Allows different nodes to be selected for calculating the maximum.
Fixes threshold bug

Needs to be set to np.float32.
Restores missing routines

These were in the original ImageD11 files but had been removed. They
are now needed for the new workflow.
Changed two-theta limits

The maximum two-theta was too large, allowing many overlapping rings to
distort the result.