Reference corrections to point, convert to meters

[scottstanie]

• Changed units for ionosphere/troposphere to meters (instead of radians)
• We are not subtracting the reference point value for each raster
• We are returning the ReferencePoint tuple to avoid needing to read the plain text file

inspecting an output looks like the ionosphere is centered around 0 now (instead of offset by ~2)

• Also I added units to the raster band metadata:

  $ rio info scratch/historical/atmosphere/troposphere/20170217_20170301_tropoDelay_ECMWF_LOS_comb.tif | jq .units
  [
  "meters"
  ]