Fix reading/writing HFSS Touchstone files using the 'power' S-param definition

[jhillairet]

Fixes #804

• If the s_def parameter is not explicitly passed (ie s_def=None), the assumption of power is delayed after have read the touchstone data.
• If HFSS-generated, s_def is set to the default HFSS setting only if s_def has not been passed explicitly. Otherwise if uses the passed s_def

The following will work:

ntwk_orig = rf.Network('hfss_file.s1p') # HFSS generated file
self.assertEqual(ntwk_orig.s_def, S_DEF_HFSS_DEFAULT)
        
ntwk_orig = rf.Network('hfss_file.s1p', s_def='power') 
self.assertEqual(ntwk_orig.s_def, 'power')

where S_DEF_HFSS_DEFAULT is a new constant, currently defined as 'traveling' (while i'm still not sure about the real meaning of "Generalized" in HFSS...)

• For writing a Touchstone file, it now assumes the user knows what he is doing and i've removed the s_def check/warning before writing a file. So now, importing a 'power' Touchstone file (passing s_def='power' when reading) and writing the Touchstone file should lead to same results.