Endpoint to retrieve min/max values for criteria ranges

[ConnectedSystems]

Endpoint: /criteria/ranges

Example return:

{
  "Depth": {
    "min_val": -19.75835,
    "max_val": -0.09198824
  },
  "Benthic": {
    "min_val": 13,
    "max_val": 15
  },
  "Geomorphic": {
    "min_val": 21,
    "max_val": 24
  },
  "Slope": {
    "min_val": 0.012136885,
    "max_val": 48.634396
  },
  "Turbidity": {
    "min_val": 1,
    "max_val": 454
  },
  "WavesHs": {
    "min_val": 0.2369229,
    "max_val": 2.5359628
  },
  "WavesTp": {
    "min_val": 1.9433917,
    "max_val": 9.299267
  },
  "Rugosity": {
    "min_val": 0.1420705,
    "max_val": 9.601867
  }
}

This PR has changes that were more focused on tidying things up, moving code around and adding docstrings, so it ended up not being so nice, neat, and tidy. Sorry folks, not what I usually do.

@arlowhite Not sure what to do on the frontend with values like 2.53 - use a value rounded up to nearest whole number?

[PeterBaker0]

Looks good - hard to review thoroughly without actually testing it out/but have scanned through highlighted areas. Thanks for the inline comments and doc strings.