Validate step values on inputs

[louispt1]

Description

This change adds step-value validation to the scenario updater. These changes were separated from the scenario updater PR because they may require more discussion.

Users' workflows could certainly be affected. For example now if I try:

curl -X PUT http://localhost:3000/api/v3/scenarios/2729350 \
    -H "Content-Type: application/json" \
    -d '{
      "scenario": {
        "user_values": {
          "climate_relevant_co2_biomass_liquid_present": "50.05"
        }
      }
    }'

I will get a response:

{
  "errors": [
    "Input climate_relevant_co2_biomass_liquid_present value 50.05 must align with step size 0.1. Nearest valid values: 50.0 or 50.1"
  ]
}

• We ought to update the documentation for the API regarding step values, after we decide exactly how to manage them

closes #1606

[mabijkerk]

@noracato you marked me as a reviewer, I'm not to sure how to review this? Do you want me to test this new functionality?

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