Allow data for a single indicator to come from multiple inputs

[brockfanning]

This is a low-priority feature without a clear user requirement, but I wanted to post this in case it's interesting. I think the main use-case for this would be something like:

• The main data input is difficult to edit and lacks some disaggregation (such as data from a third-party web service or something like that)
• The developer would like to have a local CSV file with additional data rows to add the necessary disaggregation
• So the developer adds a CSV input, expecting the CSV data to be "concatenated" onto the other data

Since it's a minor code change I thought it would be worth posting. This would definitely need testing though, in case it has any unforeseen consequences.

[brockfanning]

The user requesting this doesn't need it anymore, so I'll close this to save development time for more important features.

[brockfanning]

Re-opening this since it was discussed that it may be useful. I suspect it needs to be tweaked to work with the latest code though.