asserts for the arguments of parcel

[trontrytel]

Do we want them?

For example:
outfreq == int
gstdev != 1
(see test_cmdline)

[djarecka]

outfreq == int - I believe that the help and the python error message (clearly says that 1. is not int) is enough
gstdev != 1 agree

[trontrytel]

do we want to add a function (in parcel.py) checking asserts on some of the arguments of parcel?

[djarecka]

done for gstdev, and some other.
still needed for microphysical and chemical variables.