Add duration prop to compute startDate for nihms loader

[rpoet-jh]

We need to add a duration prop to the harvest properties of nihms loader so that the startDate parameter can be computed using the duration value at runtime of the nihms loader. Currently, the startDate is a property in harvest properties, but it is in the format MM-YYYY. The nihms loader runs on a schedule with no user input, so having the duration prop to compute the startDate is cleaner.