improve performance of Coaxial.from_attenuation

[eendebakpt]

The Coaxial.from_attenuation check on input sizes converted the Frequency argument to an array with a very slow loop including a call to __getattr__ on each element. We replace the conversion with a lookup of Frequency.f

Benchmark:

freq = skrf.F(.15,1.7,801, unit='GHz') # fails   
att_db_per_meter = 3.0
medium=Coaxial.from_attenuation_VF(freq, att=att_db_per_meter, VF=.69)

Master: 3.5 seconds, this PR: < 0.1 seconds.

@FranzForstmayr

[jhillairet]

Nice catch!