Standardize and validate variable scalars, ints, others for to_windows, shift, round, and similar methods

[tariqshihadah]

Multiple methods take inputs of either a single scalar/int/other value, an array of the same, or a reference to a column label in the events dataframe whose values get pulled to satisfy the input. Validation of this should be pulled out in a separate hidden validation method that can generically validate any of these input types, returning a standardized array.

[tariqshihadah]

Added a generic method and applied to to_windows and shift for application test case.