Removed unnecessary imports
Invalidate assignment of NXroot objects to other groups.

Also fixed file setattr bug
Added overplots to one-dimensional projections

A checkbox in the projection tab is made visible when an overplot is
possible.
Added "Duplicate" and "Remove" to file menu

The first allows copying of whole trees and the second clarifies the
difference between deleting data in a file and removing a file from the
tree.
Fixes some file save issues
Adds a new projection panel

Adds a new dialog that makes it easier to perform projections over
arbitrary limits