Customisable column mappings on import

[willowo]

Users may want to be able to display data which comes in different schemas by being able to map the columns headings in their .csv to the relevant role (e.g. 'importer', 'exporter', volume of trade') when they import the data into TradeMapper. This would be really useful as data comes in so many different schemas. Currently TradeMapper works with three different schemas (CITES, ETIS, Seizures) with column headings which are not relevant to data coming from other sources e.g. Customs data.