rename omega parameters to frequency

[oskooi]

#919 introduced an omega parameter for the frequency for various Python and C functions such as get_epsilon_point, get_array, output_epsilon, etc. Since omega does not refer to the angular frequency (which contains a 2π factor), it should be renamed to frequency. This is consistent with other Meep functions which have a frequency argument. However, to maintain backwards compatibility, omega should continue to be supported for those functions which currently have an omega parameter (but still removed from the documentation).

[stevengj]

(I think the only way to implement this is to support both arguments, and give a deprecation warning if omega is passed.)