Confusing error for test inversions with bc_freq=monthly

[joe-pitt]

It is often useful to run a test inversion using a few days/weeks of data before launching into the real thing. However, if you have set bc_freq=monthly then the run fails with a confusing error:
ValueError: zero-size array to reduction operation fmax which has no identity

It would be good to make the run fail immediately after launch with a more descriptive error telling the user not to use bc_freq=monthly if the inversion period is less than one month