Add telescope constraint function for the Telescope class

Release notes

Adjust the way telescope constraint works. These should be defined at telescope level, with default airmass constraint in case of non-existence.

Problem to solve

Different telescopes have different constraints, which should not be in the main windows calculation function.

Proposal

Add telescope_constraint function to the PTO.telescopes.Telescope class, which would adjust the observable arrays in the PTO.transits.Event class.

Intended users

Everyone who uses PTO.