Use explicit imports in modules `elec.ports` and `thermo.ports`

Description

Remove automatic import function in __init__.py files of elec.ports and thermo.ports.

This makes the module content clearer, and allows features like autocompletion in IDEs.

Notes

• The same change could be made for fluid.ports, but was not included in this revision, to avoid interfering with ongoing developments. To be done later, then.
• We should use explicit imports in all modules (ports and systems, in particular).