Fix inconsistency for disabling frequency matching for generator channel

[paniash]

In the docstrings for the add_readoutconfig() function in asm_v2.py, it is mentioned that in order to disable frequency matching, one must set gen_ch=None. However in practice, passing gen_ch=None explicitly raises a KeyError.

With the current implementation, this is checked by passing the parameter onto a dictionary par which then is used to check the existence of the key gen_ch instead of its value None.

With this fix, the docstring and code agree with each other i.e. setting gen_ch as None now disables frequency matching.

It also replaces the KeyError with a RuntimeError that names the readout and generator channels.

[meeg]

Good catch. I think the original intent was that if you didn't want matching you would omit gen_ch, but the docstring was telling you to do gen_ch=None which would not work. Now either works. And the helpful error message is a nice addition.

Thanks, and good to see you again since #293.