Created fundamental implementation of elementsets and nodesets in all models

Anne Poot requested to merge implementation-basis into master

As little changes as possible have been made to make this work. The elementset and nodeset are iterable, so all original functions used on the lists still work on these sets, which minimizes the amount of changes needed The main changes are in initmodule, where all mesh-reading procedures have been heavily modified, and the configure function in the models.

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