Add option to provide custom names for unit models, and arc.

[avdudchenko]

This adds ability for a user to specify custom names for unit blocks and unit arcs by providing a unit_model_display_names option to Connectivity module. The unit_model_display_names should be a dict that maps block names and arc to desired display names, for example.
The bloc names should be unique, while arcs do not have to be unique, as in the example bellow:

map_dict= {
        m.fs.M01.name: "user_M01",
        m.fs.H02.name: "user_H02",
        m.fs.F03.name: "user_F03",
        m.fs.s01.name: "connector",
        m.fs.s02.name: "connector",
    }
conn = Connectivity(input_model=model, unit_model_display_names=user_name)
conn.show() 

Will produce the following output:

Arc naming is only supported in Mermaid graphs, as it's done on the fly during generation, we need to add support for D2, but I am not sure how to do it as I only use Mermaid atm.