Add access functions for sparse access to arrays

[chrbertsch]

Disucssion at FMI 3.0 regular design meeting: There are use cases to access only individual elements or parts of an array. For get/set functions this might be simple (but would double the number of API Access functions), but would get complicated (but useful!) for partial derivatives. This can be implemented in FMI3.1, can be ignored by an FMI 3.0 importer

[andreas-junghanns]

web design meeting: Pierre: Is this to support actual sparse matrices, or sparse access to full matrices? Christian: the later. Pierre: The pointer-based access would be sufficient for that (see #515). Andreas: this would be a 3.1 addition with another API call that is ignoreable for 3.0 importers. Pierre: ... at least for CS, for ME some more thoughts are needed.