Validation tool

[stufraser1]

Creating the mapping files has been termed 'fragile' - easy for users to make an error when editing them, and it being hard to locate the error causing a transform to fail. This will be helped by error handling, but also a mapping file validator.

This should validate that all sections of mapping file are present and recognised:
-[ ] Mandatory commands (e.g., types, forward, etc.) are included and with correct spelling
-[ ] Whether optional commands (reverse) are included and with correct spelling
-[ ] That each data type is recognised
-[ ] That field names are recognised (requires a lookup to the model field names)
-[ ] That mapping files contain the fields contained within the data file to be transformed?
-[] indentation correct
-[] unexpected charcaters e.g. Unexpected character in 'GeogScheme1 = "CRESTA"': '=' at position 13

See #14 for some issues identified

To be added to...