missing value management

[david-i-berry]

• the mapping templates need to be updated to include specification of missing values.
• review of code required to make handling of missing values more robust.

[david-i-berry]

Add valid missing values to documentation, see:

https://github.com/wmo-im/csv2bufr/blob/42bb6e93ed461490c40f0155a429752feb598872/csv2bufr/__init__.py#L47