fix picket fence gui

[alanphys]

@jrkerns While doing routine QA I came across the following issue:

File "/usr/lib64/python3.8/tkinter/init.py", line 1895, in call
return self.func(*args)
File "/home/alanphys/Programs/pylinac/pylinac/py_gui.py", line 118, in analyze_pf
pf.analyze(tolerance=self.pf_tol.get(),
TypeError: analyze() got an unexpected keyword argument 'hdmlc'

I know you want to deprecate the gui but there are still some folk using it particularly in our company so I thought I would make this quick fix. I've deleted the hd_mlc checkbox and added a combobox with the different mlc types and passed the selected value as a parameter to the picketfence.init

I've only tested on HD mlc, but the others should work as there are no changes to the underlying code. The new MLC Arrangement is certainly much more flexible.
Regards
Alan

[coveralls]

Coverage decreased (-0.03%) to 80.332% when pulling dc5b07c on alanphys:fixpfgui into 90f261b on jrkerns:master.