Make config files parsing more user friendly

[wolfidan]

Currently, the code does not give much information on why the parsing of config files fails.

We plan to improve that by giving more information on the exact location of the parsing error and give hints to the user on why it fails.

We could also add the option for the user to validate the parsed config files by printing the full parsed configuration in a tidy way and wait for user input before continuing with the processing.

[wolfidan]

For the moment I just added some info on the directory in which pyrad searches for radar files, see commit ce67680