Allow multiple definitions for parameters where .multiple = True

[ndevenish]

Saw user try

scan_range=1-600,900-1600

This doesn't work, because it needs to be passed as

scan_range=1-600 scan_range=90-1600

This actually fails with a nonsensical error, so perhaps there is a problem with the parsing interface also inside dials

[dagewa]

Assume this refers to the integrate step. This is a general problem for all parameters where .multiple = True in the PHIL definition. A widget that allows duplication ("add another row") would be a reasonable solution.