Improve consistency of ITime support #123
Description
The schedule constructors accept start_date as an ITime, and then converts it into a DateTime internally. The NetCDFWriter also accepts start_date as an ITime, but it does not convert it. Then, when writing to the file, it converts the integrator time from an ITime into a float, and then into Dates.Milliseconds, and then attempts to add it to the unconverted start_date. This leads to an error.
It should also be able to handle cases where the ITime periods are different.