Switch away from `pint` time units

**Describe the bug**

The `pint` time units are not calendar aware and use _average_ lengths for months etc. The critical `update_interval` is stored as a `pint.Quantity`. We need calendar aware time calculations, which are probably most easily achieved using `np.datetime64`and `np.timedelta64`. There doesn't seem to be a simple conversion between `pint.Quantity` and `np` without writing some kind of interpreter.

**Expected behavior**

Months should not be 30.4375 days.

I think `pint` is _only_ currently used for the `update_interval`. It seems easiest to replace it with using `np.timedelta64` and a config like `[1, 'month']` , with a slim wrapper to convert a subset of time intervals to e.g. `np.timedelta64(1, 'M')`.

Start by locating the update_interval configuration and every current use of pint in the repository, then inspect how intervals are parsed and consumed. Replace the existing time-unit handling with calendar-aware intervals while preserving the configuration shape described in the issue; done means months are not treated as 30.4375 days and update intervals continue to work.

Written by the indexing model from the issue text.