Migrate watertap.ui.fsapi to idaes_flowsheet_processor

[lbianchi-lbl]

Resolve: #1517

Changes proposed in this PR:

• Remove code in watertap/ui
• Update imports: watertap.ui.fsapi -> idaes_flowsheet_processor.api (we might want to just use the top-level package instead; see watertap-org/idaes-flowsheet-processor#FIXME)
• Add idaes-flowsheet-processor (currently from the repo URL https://github.com/watertap-org/idaes-flowsheet-processor) as an optional dependency under the testing target
• Refactor optional dependencies by removing the dev target and splitting its contents between the testing target and requirements-dev.txt

Before merging

• Update idaes-flowsheet-processor requirement currently pointing to Add pytest plugin for project-agnostic testing of flowsheet interfaces idaes-flowsheet-processor#6

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