Add monthly and yearly to period generator

[lucarlig]

The transaction limit is 1200 bytes so uploading an array of periods would require a lot of requests for long SLAs with many periods.

the idea is to implement a generator that generates the timestamp giving a start time, number of periods, and periods type.
minutes and weeks are just simple multiplications. To implement months and years period types need to consider the calendar rules around Feb 29th and 4th years.

[lucarlig]

reinstated generator with an enum for monthly, yearly, and custom. monthly and yearly still to implement