Increase date documentation, coverage and functionality #434
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Improves
mir.datedocumentation and coverage.Also adds
addMonths,addQuarters, andaddYearsforYearMonthandYearQuarter. The difference with theaddfunction is that they return new values instead of references. I would be open to changing the naming, e.g. calling itadvanceand make it a template likeadd. Or if there's some other way to get this functionality that I'm not thinking of, I would be open to it.