Updated base classes and fixed exception

There was a syntax error in a StandardError exception statement.
Added use of libtiff library

The standard Python Imaging Library cannot read some x-ray detectors
that produce Float-32 data. This version attempts to use the libtiff
library, but reverts to PIL if there is an ImportError.
Clear checkboxes on a new plot

Sometimes, the old checkbox state (e.g., logplot) was not reinitialized
with a new plot, which could cause problems, e.g., if a log plot isn't
possible.
Moved import so that it is not a dependency on startup

It will not be a problem if the pyspec library is not installed, unless
you try to import a SPEC file.
Initial implementation of least-squares fitting

A new Fit Dialog is an option in the Data menu or by right-clicking on
a data item. This allows the addition of several functions, as defined
in the nexpy/api/frills/functions directory. Still in the early stages
of development - parameters can be fixed but not limited or bound yet,
and the results of the fits need to be displayed. However, the results
of a fit can be saved to a scratch NXentry group in w0 on the tree.