BUG: longitude orbits not working correctly or consistently

[aburrell]

Describe the bug
Longitude breaks for orbits are not robust and frequently break with unexpected results.

To Reproduce
DMSP is a good example for this. The longitude key is 'glon'. This will break whether you use longitude in the +/- 180 range or 0-360 range.

Expected behavior
I expect one orbit to have the whole range of longitudes. Bonus: latitude breaks. Currently 'polar' will break with the hemisphere changes. This could be easily adapted to break the orbit at the northern or southern peak of an orbit.

Desktop (please complete the following information):

• OS: OS X Big Sur
• Version Python 3.8
• Other details about your setup that could be relevant: develop branch of pysatMadrigal

[aburrell]

Another idea could be to have the orbital period in the Instrument file and allow the user to use the satellite orbital period with a user-specified starting point.

[rstoneback]

The longitude orbit iterator currently only looks for negative gradients in longitude, aka it is expecting longitudes to go from 0 to 360, rather than from 360 down to 0. The code needs an option for the type and value of the gradient to trigger off.