add documentation how to get rid of xrayutilities parsing messages

[dschick]

when reading SPEC files with the xrayutilities backend, there will be a lot of messages printed while parsing.
This can only be changed directly when loading the module with
xu.config.VERBOSITY = 0
follwoing the description at
https://xrayutilities.sourceforge.io/examples.html#user-specific-config-file

I suggest to automatically change xu.VERBOSITY following the pyevaldata loglevel, because its is not possible to make it directly accessible to the user.