Add a Json to Specification object converter

[alelom]

Currently, any Json data containing a Specification or a Condition and that adopts the Json-schema described in the repo:
https://github.com/CIHDigitalConfigurator/CIH_Specifications_Schema

can be read using the BH.Engine.Serialisation.FromJson() method. However, the returned object is always a CustomObject, and some manual adjustment is needed to convert it to a C# Specification or Condition:

Implement a custom deserialiser to solve this.