flow + dt or times array, but not both

[sfield17]

When passing an array of times, models require the user to supply something for f_low. Probably the behavior should be one of

1. times (as an np array)
2. dt and f_low

but input like times and dt, or times and f_low, should raise an error. if times and f_low are allowed, f_low should only be checked (starting time is a frequency lower than f_low).