PyDASSH improvements

List of bugs and improvement suggested:

• Make wire wrap specifications optional in detailed assembly definition
• Fix the following type of errors:

  File "/Users/nstauff/PyARC/PyDASSH.py", line 646, in prerun
    tmp = getattr(dassh.setup.write_to_csv_file, k)
AttributeError: 'NoneType' object has no attribute 'all'

• Provide alternative to gap_material = sodium
• The following error message is unclear because low-fidelity models don't seem to be accepted - it appears like a full model would be requested. ERROR: Need at least one pin bundle subassembly per assembly; it can be specified, inherited, and /or modeled using the low-fidelity model (error in Assembly "cr")
• the following error occurs because of use of ' instead of " - both should be allowed.

ERROR: Incorrect path to GAMSOR output files /Users/nstauff/Documents/CORE_STUDIES/anl_mr/'dif3d_gamsor_0'

• not clear why the sub_assembly.coolant_volume_frac needs to be specified when not needed.

  File "/Users/nstauff/PyARC/PyDASSH.py", line 1029, in prerun
    sub_assembly.coolant_volume_frac.value))
AttributeError: 'NoneType' object has no attribute 'value'

• not clear why I get the following error when using inherit_from = Fuel_SA for sub-assembly within fuel region:

ERROR: If multiple pin bundle subassemblies are specified in DASSH input, all must be identical (error in Assembly "fl")