Adding non-exact grouping

[martinju]

This PR adds non-exact grouping, i.e. the kernelSHAP method of sampling n_combinations of the 2^no_groups different groups available to use in the computation. It is essentially a duplicate of the code for features.

Other edits:

• Restructured the if-else checks for whether to switch to exact when a too large number of n_combinations is specified.
• switched to update=F for one test against a disk stored file

[JensWahl]

Looks good to me.