What changes were proposed in this pull request?

A foundation for the future GQL query-engine: a graph schema. I'm going to use it to determine possible paths and split query to parts at the stage of query analysis.

At the moment:

• Schema classes
• Helpers: schmea and schemaDOT for the PropertyGraphFrame

JVM only for now.

Why are the changes needed?

Close #829

How it looks like?

Schema:

Property graph schema:
Vertex property groups (2):
  - movies
  - people
Edge property groups (2):
  - likes: people -> movies
  - messages: people -> people

DOT-schema (can be imported to NetworkX or any other tool):

digraph SchemaGraph {
  "movies";
  "people";
  "people" -> "movies" [label="likes"];
  "people" -> "people" [label="messages"];
}