Fixed indexing with NXfields

This now allows an NXdata slice index to contain an NXfield as well as
a floating point number. In some scripts, the slice is defined by
operations on NXfield. This allows that without requiring that the
field be converted to a floating point number first.
Added read_parameter to BaseDialog

Returns a scalar value from an NXfield. At the moment, it is forced to
be float32 - this should be changed.
Example of plugin menu

A simple example to analyze deta in chopper.nxs, which is one of the
example files.