datastorage function saveToFile fails if only results of part of calculations are available

[m97]

I have noticed that a saveToFile function present in datastorage.py is very inflexible in regard to saving results of calculations. In my opinion, not all calculations are needed for every run. For example not running interface analysis will result in empty variable interNodePairs which will cause failure of the saveToFile function. This could be avoided by checking if given data are available before saving them.