Avoid implicit changes to ict and space counts in thermal calibration

[carloshorn]

To avoid an implicit change of the input arrays, I would move the interpolation of missing/wrong data in the ICT and space counts to the POD/KLM specific reader method get_telemetry or use a copy instead of changing the input in-place.

https://github.com/pytroll/pygac/blob/8a7f11edac14e44f3caf8d6f23cf095ff77ec17e/pygac/calibration.py#L409-L425

[sfinkens]

I like the idea to move this to the POD/KLM specific get_telemetry methods. There are probably a number of methods in pygac which change data in-place and would need a refactoring. This one's a good start!

[mraspaud]

Sounds good, go ahead!