Corrected interface to idealenthalpysingle

[morteham]

No description provided.

[vegardjervell]

I discovered a unittest failure that was introduced in #135, which gives the test output

.............F................. Detected mix of parameters for simplified RDF and non-simplified RDF
.................................................
......................................................
Time:         9.718 seconds
  
Failure
 in: 
test_saturation_suite.test_vlle_binary
  Location: 
[test_saturation.pf:73]
AssertEqual failure:
      Expected: <0.85927669469936900E-2>
        Actual: <0.83983073148142483E-2>
    Difference: <-0.19445963217944173E-3> (greater than tolerance of 0.10000000000000000E-7)
  
 FAILURES!!!
Tests run: 133, Failures: 1, Errors: 0
, Disabled: 0
Note: The following floating-point exceptions are signalling: IEEE_UNDERFLOW_FLAG

It's unrelated to this PR, but should probably be looked at.

Also: I've taken the opportunity to add a pytest suite, with a module where we can easily add tests when we fix issues in the future, to hopefully improve robustness over time and help us avoid re-introducing issues in the future :)