Configure watertap as an optional dependency

[lbianchi-lbl]

• If/when Moving cyipopt solver adapter code from watertap-org/watertap to this repository #6 is complete, reaktoro-pse will cease depending on watertap as a required dependency
• However, we'll likely still rely on importing watertap for non-essential purposes, such as Jupyter notebooks
• Therefore, we need to set things up so that:
  • watertap is available and can be installed easily where needed (e.g. by bundling it together with the Jupyter dependencies so that they can all be installed in the environment(s) where the notebooks are run)
  • Everything else works (i.e. there are no test failures) if watertap is not importable
  • This will most likely require adding another CI check branch so that tests are run in environments both with and without watertap installed