Support dimensionless units of measurement

[JR-1991]

As higlighted in #56 the current implementation does not allow to add dimensionless units. This PR fixes it by providing a way to add this type of unit. The resulting Unit object will only contain the name dimensionless with an empty list of bases. In addition, the corresponding Unit(dimensionless_unscaled) will be added to the _unit attribute.

Example

unit = Unit.from_string("dimensionless")
print(unit.json())

Output

{
  "my_unit": {
    "id": "6a632e6b-a6bf-4a63-952f-ce037206c6b8",
    "name": "dimensionless",
    "bases": []
  }
}

Closes #56