provide possibility for user-defined gapfilling function used after uStar filtering

[bgctw]

Currently sMDSGapFillAfterUstar calls .self$sMDSGapFill. If a user wants to modify the gapfilling, e.g. by including more environmental variables, then he has to recode the entire uStar-Filtering/Distribution procedure.

Implement an argument fGapFill so that the user can provide its own modified gapfilling function to sMDSGapFillAfterUstar.

[bgctw]

We are in an object-oriented setting, which already provides the tools to achieve this. Instead of calling a different function than .self$sMDSGapFill in sMDSGapFillAfterUstar, just create a drived class, wehre you override the sMDSGapFill method to your needs.