Add ports

We should allow to add in ports to the nodes classes (all ports need to be billable) so that they can have properties attached to them (same for relations). These properties should allow to add metadata like 'wavelength' for flourophores , or 'strength' for relations. Optionally the should have a schema attribute. Structures should be disallwoed? What about lists of values? Also disallowed rather?

Start by locating the Python node and relation class definitions and reviewing how their properties are currently represented. The issue leaves port billability, metadata, schemas, structures, and lists as open design questions; done requires a settled design for these cases and agreement on the resulting model changes.

Written by the indexing model from the issue text.