Fix spelling suggestions proposed by Codespell

[bobleesj]

Many are suggested by Codespell:

doc/manual/html/pdfgui.html:416: extened ==> extended
doc/manual/html/pdfgui.html:2551: delt ==> dealt
src/diffpy/pdfgui/control/pdfdataset.py:232: gerneral ==> general
src/diffpy/pdfgui/control/pdfdataset.py:377: proccess ==> process
src/diffpy/pdfgui/gui/DesignNotes.txt:6: chaning ==> chaining, changing
...
src/diffpy/pdfgui/control/fitting.py:76: accurancy ==> accuracy
src/diffpy/pdfgui/control/fitting.py:203: hierachy ==> hierarchy
src/diffpy/pdfgui/control/fitting.py:262: struc ==> struct
src/diffpy/pdfgui/control/fitting.py:263: struc ==> struct
...

Resolve them.