SF: Add new mode to average

[t-b]

Average so that each selection is one input, handle x-axis mismatch.

[MichaelHuth]

• Extend avg operation to support also select input data either single or as array of selects with already existing modes. data() is then applied implicitly.

• Add new mode that accepts an array of selects with 2 or more entries. All entries have the same nubmer of datasets. Average over first entries (with support for different x-axis (already in average implementation)) then second entries and so on.