Fix masked input data handeling in index functions
The handeling of masked input data needs to be reviewed for all of the index functions.
Here, we need to the decide on the expected behaviour of each index function. We need to decide that either, if there is one masked grid-cell the the output should be masked for that grid-cell or if there should be some limit for how many values that can be masked and then how this can be used in the computations. E.g., there could be some pre-processing step that checks the quality of the data and masks all values for that grid-cell which do not fulfill the conditions etc.