Add resource for basic and acid PKA calculations

[khyurri]

This PR fixes Issue #709

Added new resource "/indigo/pka",
Currently for correct pka calculation user is required to pass pka_model_build object:

{
    sdf: string,
    max_level: int,
    threshold: float
}

Example can be found in this test: https://github.com/epam/Indigo/pull/789/files#diff-bd881ed8307e3a0d774f9e48c79acb01efa5cb3a28c0fa766ba82755dbe4a6f1R193

[khyurri]

@IuriiPuzanov

Could you please clarify, why I always get pka acid and basic 100%.

I used this example for implementations: https://lifescience.opensource.epam.com/indigo/api/index.html#pka

Example request:

{
  "data": {
    "type": "pka",
    "attributes": {
      "compound": {
        "structure": "C1=C(C(=C(C(=C1Cl)Cl)Cl)O)Cl",
        "format": "auto"
      },
      "pka_model": "advanced",
      "pka_type": "acid",
      "pka_model_level": 0,
      "pka_model_min_level": 0
    }
  }
}

Response:

{
  "data": {
    "type": "pkaResult",
    "attributes": {
      "mappings": [
        {
          "index": 0,
          "symbol": "C",
          "value": 100
        },
        {
          "index": 1,
          "symbol": "C",
          "value": 100
        },
        {
          "index": 2,
          "symbol": "C",
          "value": 100
        },
        {
          "index": 3,
          "symbol": "C",
          "value": 100
        },
        {
          "index": 4,
          "symbol": "C",
          "value": 100
        },
        {
          "index": 5,
          "symbol": "C",
          "value": 100
        },
        {
          "index": 6,
          "symbol": "Cl",
          "value": 100
        },
        {
          "index": 7,
          "symbol": "Cl",
          "value": 100
        },
        {
          "index": 8,
          "symbol": "Cl",
          "value": 100
        },
        {
          "index": 9,
          "symbol": "O",
          "value": 100
        },
        {
          "index": 10,
          "symbol": "Cl",
          "value": 100
        }
      ]
    }
  }
}

I took this compound from here.

Also, could you please clarify, how rdkit calculates exp_pKa for whole molecule. Do we need same functionality in REST service?