Remove clock_generator_initiate() from nidigital API

[sbethur]

Description of issue

Clock generator can be started using clock_generator_generate_clock() and clock_generator_initiate(). Both are pretty similar; clock_generator_initiate does not accept the frequency parameter, it should be set earlier via clock_generator_frequency property.

In the spirit of The Zen of Python:

Explicit is better than implicit.
There should be one-- and preferably only one --obvious way to do it.

we should remove clock_generator_initiate().