Update the Expiration.aerosols calculation to be based on diameter, not position

At https://gitlab.cern.ch/cara/cara/-/blob/b8b32284/cara/models.py#L226 we only apply the mask efficiency for certain particles. At present, we do this by knowing that particles >3 micron are at index 2 and above. We can simply change this check to those particles which have a diameter of greater than or equal to 3.

Add a test which sets up an Expiration activity with more than two particles with a diameter >3 to confirm that the filtering is correctly being applied.