Remove kwargs in feature_select

[gwaybio]

Related to #48 (Maybe can be completed at the same time)

kwargs are very dangerous in feature_select and provide no benefit in this specific case. Just make them function arguments.

It prevents the case where operation = "correlation_threshold" and operations = "correlation_threshold". Both of these will not throw an error, but since the correct function argument is "operation", operations = "correlation_threshold" will actually perform feature selection with the "variance_threshold" default. This is not good!