Verify changes with older versions of Dyalog

One concern that has risen is values of results of some expressions might change with different versions of dyalog, particularly floating point numbers.

To prevent this from happening another variation can be added to RunVariations (in ./testfns.apln) that spins up an older version of Dyalog and verifies the outputs again. (actual implementation needs to be discussed)

TODO: Need to check the feasibility of this with @dyaandys. Need to re-check the need for this type of tests with @mkromberg.

Thread is open for discussion and suggestions.