Make MSA a scientific variable

[fpavogt]

Describe the change
Currently, msa is only a keywords applied to the functions issuing metars. However, cropping very high measurements prior to feeding them to the ampycloud algorithm does impact its outcome, by allowing for thinner slices.

To ease this, we need to create two new parameters: MSA and MSA_HIT_BUFFER.
The former is meant to be the official MSA value of the site, and the latter is a (positive) altitude buffer above which hits will not be considered by the ampycloud algorithm.

In p[articular, there should be a test setup to verify that MSA_HIT_BUFFER is postitive.