Generate Mermaid images directly

[dangunter]

Instead of asking the user to run scripts etc. themselves, use mermaid-cli to generate images directly.

Also simplify API generally and specifically for Mermaid diagrams.

[dangunter]

fyi, to use in a script/module:

from idaes_connectivity import MermaidImage

# build your model...
model = build_model()  

# create PNG file for model in 1 line!
MermaidImage(model).write("model.png")