Added
- The `asilib.acknowledge.acknowledge()` function. It should be called by each loader to print that ASI's acknowledgment statement either when 1) the first time the user calls the function, or 2) when it has been a month since it was last called.
- Tests for `asilib.acknowledge.acknowledge()`.
- Added the call to `asilib.acknowledge.acknowledge()` in `asilib.asi.rego()`, `asilib.asi.trex_rgb()`, and `asilib.asi.trex_nir()`.