fix #9216: more consistent units for Period arithmetic

This fixes #9216 by defining `Period` arithmetic consistent with the usual rules for dimensionful quantities.  e.g. `Period + Real` is no longer defined, whereas `Period / Period` gives a `Real` value.   I also expanded the vectorized definitions to make them more consistent with other `Array` arithmetic, and added addition/subtraction of periods with date ranges.

cc: @quinnj