Improve DKG assembly of physical constants

[cthoyt]

This PR does the following:

• Adds initial values to physical constant nodes using debio:0000042 as a property (newly minted)
• Adds formulas to physical constant nodes using debio:0000043 as a property (newly minted)
• Adds symbols as synonyms for physical constants
• Small cleanup of synonym type predicates in other places

When you call the API you now get

curl -X 'GET' \
  'http://localhost:8771/api/entity/wikidata%3AQ13534105' \
  -H 'accept: application/json'

{
  "id": "wikidata:Q13534105",
  "name": "electron magnetic dipole moment",
  "type": "class",
  "obsolete": false,
  "description": "spin of an electron",
  "synonyms": [
    {
      "value": "electron magnetic moment",
      "type": "oboInOwl:hasExactSynonym"
    },
    {
      "value": "\\mu _{\\mathrm {e} }",
      "type": "debio:0000031"
    }
  ],
  "xrefs": [
    {
      "id": "nist.codata:muem",
      "type": "oboinowl:hasDbXref"
    }
  ],
  "labels": [
    "constant",
    "wikidata"
  ],
  "properties": {
    "debio:0000042": [
      "-0.0000000000000000000000092847647043"
    ]
  }
}

[bgyori]

Great, thanks!