UHS config param validation

[Lars Butler]

The UHS calculator only really adds 1 new config param:
UHS_PERIODS (ordered list of floats, >= 0.0, no duplicates)

However, we need to some additional parameter checking:
1) The INTENSITY_MEASURE_TYPE is always assumed to be SA for UHS calculations. Thus, IMT is not a required param; if it is defined, the UHS jobs should ignore it.
2) PERIOD is typically a required param for calculations with IMT=SA. For UHS calculations PERIOD (which is a single float value) shall NOT be defined. Instead, UHS_PERIODS (a list of float values) is required instead. (Note: PERIOD and UHS_PERIODS are used in a similar as parameters for GMPEs.)

[Lars Butler (lars-butler)]

Pull request: https://github.com/gem/openquake/pull/575