Update uniform_hazard and risk_coefficient data loaders

There are longer any uniform hazard (SA) or risk coefficient (CR) data for NEHRP-2020. The data are natively computed to incorporate risk targeting into the hazard data.

• Requests to uniform-hazard.json for NEHRP-2020 should return pgauh as it does currently, however sauh should return periods and ordinates as empty arrays.
• Requests to risk-coefficient.json for NEHRP-2020 should return an error message.
  • Currently something like "Cannot read property 'id' of undefined". This could be improved to be more user-friendly, but not necessary at this stage.