Add missing CFMessage conversion methods and tests

[RobertRosca]

Should probably implement these two functions, although they're only used when saving data, which is not done yet:

• to_grib_step
• to_grib_date_time

Need to add:

• Specific tests for these functions
  • Maybe parity tests with python implementation again?

Need to investigate:

• from_grib_step application, the method seems correct however applying it results in different values from the python implementation
• Is it actually worth doing the type-dispatch method for from_grib_date_time? It's not too important, and makes the code more difficult to read through
• Better implementation of read_message? Not the most elegant solution, something better must be out there