sensorCentralFrequency data type should be a float

[rhoneyager-tomorrow]

This is mostly a documentation issue since I'm unsure if any IODA converter writes out the sensorCentralFrequency and radarPulseFrequency variables, but under the current version of IODA they can't be double precision variables. This is because IODA's VarUtils functions lack support for double-precision floating point numbers. This prevents a double-containing IODA file from being read into an ObsSpace.

Fixes:

Option A: Change the docs and make sensorCentralFrequency a plain old float.

1. https://github.com/JCSDA/ioda/blob/develop/share/ioda/yaml/validation/ObsSpace.yaml#L952 -> Double to Float.
2. Obs variable names table

Option B: IODA type system refactor. This would be much harder, though in JCSDA-internal/ioda I left branches from my attempt about two years ago.