Advanced period arithmetic in Muon ALC interface

The Muon ALC interface only allows either a single period to be analysed, or one period to be subtracted from another (in Mantid 3.9, at least). The full range of period arithmetic should be offered, as in Muon Analysis.

Example uses: (a) adding the two periods in a run, to improve statistics when determining a background, or if runs were taken with the RF/laser turned off but the DAE still in period mode, or (b) complex calculations such as (1+2)-(3+4).