Add remaining time formats

[fsaad]

In BayesianNeuralFieldEstimator, the timetype variable is assumed to be index. The more general version is

• timetype='index': Converts entries in the time column (whose type is datetime.datetime) to a discrete time index, as specified by the freq parameter.

• timetype='unix': Converts entries in the time column (whose type is datetime.datetime) to a float according to UNIX time.

• timetype='float': Leaves entries in the time column (with any numeric type) as is.

The freq should be generalized from str to str | None, since it only makes sense for timetype=='index'.