Construct Titan graph schema based on the abstract one

Continuing https://github.com/ohnosequences/puntos-y-flechicas/issues/13

- [x] define what is a schema for titan
  + set of property keys
  + set of relationship labels
  + their additional titan-specific characteristics
  + any kind of connection between vertices/edges and properties (not possible to restrict in titan)?
  + ...?
- [x] construct titan-schema from abstract-schema definitions
why a type? what's wrong with `x: VT`