Add unit to `EquationOfStateFitting`

[singularitti]

• Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines

• [ ] Tests for the changes have been added (for bug fixes / features)

• [ ] Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Fix some unit conversion problems introduced in #22. Try to solve #23.

• What is the current behavior? (You can also link to an open issue here)

• What is the new behavior (if this is a feature change)?

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

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[singularitti]

Close because we have #25.