Keep base directory in sys.path when building the docs.

This could break sphinx when srfit is installed in a development mode.
No need to keep old entries around.
Use full import path for utils.

Also fix punctuation and PDF instead of Pdf.
Rename testcase --> testoptional.

This hopefully improves readability of the optional tests.
Also prefix with underscore local in the utils module.
Decouple diffpy.Structure and pyobjcryst optional tests.
Improve warning texts when dependencies are not installed.
Delay imports until they are truly needed.