Hydrochemistry: add function for Stiff diagrams

[cecileherr]

It could be useful to add a function to create Stiff diagrams (a graphical representation of Na, Cl, HCO3, Ca, SO4, Mg in a chemical sample)

Inspiration sources:

• Existing r packages and sources: GQAnalyzer package https://github.com/khaors/GQAnalyzer (but order of the ions is not the same as in the Watina app)
• existing script in Watina app
• old Excel script still available from: http://www.riv.co.za/wv/Scripts.html

This function should work with the output of get_chem().