Add concept query schema and paths

[ChristophB]

Concept queries can use concepts from multiple repositories and allow construction of simple Boolean expressions.

[ChristophB]

• concept queries are comparable to phenotype queries
  • maybe we can reuse some of the object properties
  • we also need a new path to query for SONG functions (see also top-phenotypic-queries: SONG.getExpressionFunctions())
• implementation of concepts:
  • new schemas
  • or: new entity types that only occure in concept repositories

[ChristophB]

We decided on the following changes, which will be applied after merge of #120:

• [x] move property expression from Phenotype to Category
• [x] introduce two new EntityType enum values: 'single_concept' and 'composite_concept'
• [x] add property type to Expression:
  • type: string
  • description: 'Determines the result of a text function. For example: initial term list, processed term list, ...'
  • optional
• [x] add ConceptQuery schema, which has a concept entity. There might also be other required properties like a date

[ChristophB]

Do be discussed:

• introduce a sub class 'Concept' for single and composite concepts?