Add as_table() method with more flexibility

[taldcroft]

The current .table property on event models has the behavior of silently dropping columns that have missing values. This happens with the NULL values in the database get turned into None, which makes the column get initialized as Object dtype, and subsequently dropped. This happens with manvrs for instance.

kadi/kadi/events/models.py

Line 298 in cccaa77

drop_names = [name for name in dat.dtype.names if dat[name].dtype.kind == 'O']

A new method as_table() could address this by controlling whether the output is a masked table, or warnings are issued, or turning date strings into cxotime mixin columns, etc.