development mode file dir wrong

**Description**
Tests not running properly when installed in developer mode.

**Reproduce**
run `pip install -e .` then run tests like `python -m diffpy.pdfgui.tests.run` it will show errors.

**Trace back**
The problem is possibly located at `diffpy/pdfgui/gui/pdfguiglobals.py` and maybe other files.
See line 43:51.