Change primary import location to idaes_flowsheet_processor

[lbianchi-lbl]

At the moment, the primary import location is idaes_flowsheet_processor.api, i.e. we have:

from idaes_flowsheet_processor.api import FlowsheetInterface

However, I think this is mostly a remnant of the original package structure, and we could just get rid the api subpackage, i.e.:

from idaes_flowsheet_processor import FlowsheetInterface

[lbianchi-lbl]

@dangunter Thoughts?