Pairwise operation mode: unexpected behavior with functions that take a single second operand

[PierreRaybaut]

Pairwise operation mode:

• Unexpected behavior when using the pairwise operation mode with functions that take a single second operand (e.g. for images: difference, division, arithmetic operations, and flatfield correction)
• If only one set of operands is selected in a single group, a warning message is displayed "In pairwise mode, you need to select objects in at least two groups.", which is correct for functions that are symmetric (e.g. addition, multiplication, etc.), but not for functions that are not symmetric (e.g. difference, division, etc.).
• The warning message should only displayed for functions that are symmetric (e.g. addition, multiplication, etc.).