VariableRequest creation

Implement basic creation for VariableRequest objects.
Refactoring of code and modules

Move cyti.types.types to cyti.types.core for consistency with the
conversion module (where the internal Cython portion is named core).

Remove duplication in cyti.types.core in terms of creating
Variable[Request] objects. Now, the same code is used to initialize the
essential parts of both objects.