Filter out "n/a" values in S2I conversions

[rolfkleef]

When adding values, filter out common text strings, at least "n/a".

Perhaps make it broader: only add numerical values (as per IATI rules, although the schema allows text in target/actual/baseline values).

See what to do with non-numerical strings other than "n/a" or similar NULL texts: add to comment field?

[rolfkleef]

Filtering it out in Akvo conversions. Moving it back to Icebox to implement in S2I in general, after better defining which values to replace.