Capture of external logs and general improvements

[RoryPTB]

Changes made:

• Added support for zero precipitation when the precipitation indicator iR = 3. That is, when iR = 3, absent precipitation groups represent zero preciptation, not missing data.
• NIL reports are no longer written as an empty BUFR and the user is informed of this through a warning.
• Section 3 and section 4 mappings were not being updated correctly which caused the incorrect error Missing station height. This has been fixed.
• Test data has been updated, most notably the wis2box-data now has a barometer_height column to meet the current requirements of synop2bufr.
• Pymetdecoder and csv2bufr logs are now captured and stored appropriately in warning and error arrays which can be accessed in the webapp.
• Valid min and max values have been implemented directly into the 307080 and 307096 mapping files so that out of range errors are handled before ecCodes and are more informative for the user. Many of these valid ranges have been taken from the WMO Guide to Instruments and Methods.
• Added a verification that, if both reported, the dewpoint temperature is less than or equal to the air temperature.
• Unit tests have been added for the two quality control steps above.