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* Improved unit management by adding a unit/basis for both the
adsorbent (ex: amount adsorbed per g, kg or cm3 of material
are all valid) and loading (ex: mmol, g, kg of gas adsorbed
per amount of material are all valid)
* Separated isotherm models so that they can now be easily
created by the used.
* Added new isotherm models: Toth, Jensen-Seaton, W-VST, FH-VST.
* Made creation of classes (Adsorbate/Sample/Isotherms) more
intuitive.
* Many small fixes and improvements