Do some basic validation of uploaded CSV.

[hleumas]

First of all, validate whether it is a valid CSV file.

Secondly, there are some rules:

– all fields are mandatory with the exception of user, slackID, and VATID
– VATID is mandatory if VATPayer is true.
– invoice number must be numeric
– all dates have to be in yyyy-mm-dd format
– preTaxCost and VATLevel are in .toFixed(2) format
– If for one item at least one of serviceName, preTaxCost, VATLevel is filled, all other for the same item have to be filled too.