Added slot decorators to spin box read functions

This is to ensure that they respond to valueChanged signals emitted
during calls to stepBy.
Added a set class property

This allows an NXgroup to be set to a specific NeXus class, e.g.,
NXsample, using the nxclass property. The value has to be a valid class
(i.e., not a string, at least for now).
Added base class definitions

The NXDL files are now to be distributed with NeXpy, allowing valid
group classes and fields to be suggested when adding or initializing
data.
Added input of base classes

Now the ‘Add Data’ and ‘Initialize Data’ dialogs offer a list of valid
groups and fields as options, with tooltips added to the combo-box
items.