* setter for `PureFluid.TPX` property is added to allow for automatic
restoration of consistent thermodynamic data
* alternative to 'TP', 'TX' or 'PX' which may not uniquely describe
a valid thermodynamic state
* add unit test

This commit implements new methods for SolutionArray:
* `restore_data` can restore data previously exported by `collect_data`
* `read_csv` restores data previously saved by `write_csv`
* unit tests are added for SolutionArray's based on ThermoPhase and PureFluid

Incorporate reviewer suggestions for error messages (user feedback) in
PureFluid.TPX setter and SolutionArray.restore_data