Need support for additional grid (and catalog) forecast formats

[pabloitu]

There is a number of different formats out there, which could have support by pycsep. This include the .XML, the csep .csv format (e.g., has a column for each magnitude bin rate) and HDF5 (issue #193 and now implemented by @Serra314 and in floatcsep). pyCSEP currently supports the .dat format and a custom_function. I suggest we create a factory class to handle and link the format parsing to forecast creation, which should be called from the high-level load_gridded_forecast() in csep.init. In the factory we can have format checks (as agreed to solve #226 #225),

Same pattern could be applied to catalogs and catalogforecasts if necessary.