[BF] fix intendedFor

[arnaudbore]

More robust way to auto fill the intendedFor key in the json file.

Tests need to be re-written but should be working.

Fix #106

[bwagnerwork]

I'm having a bit of trouble telling from the code. Would this intended for fix work in a scenario where dcm2bids is run over a dataset several times (such as updating rules do to variations) or intended for single run or a clean rerun if needed?

[arnaudbore]

This should work with a clean run not a rerun. At this stage, I check if there is any intended for then I check the rest of the images that are going to be converted and see which one could fit. If an image has already been converted it won't work... good point. 👍