Hydrochemistry: add function for Maucha diagrams

[cecileherr]

It could be useful to add a function to create Maucha diagrams (a graphical representation of the major cations and anions in a chemical sample)
(see also this discussion: inbo/habnorm#138)

Inspiration sources:

• Existing r packages and sources:
  => Oviz package: https://github.com/fkeck/oviz/
  => GQAnalyzer package https://github.com/khaors/GQAnalyzer (but order of the ions is not exactly the same as in the Watina app)
  => maybe this? http://www.dwa.gov.za/iwqs/water_quality/NCMP/av/Maucha_map.R.txt
• small r function adapted from Oviz package for Habnorm project (using ggplot2)(https://github.com/inbo/habnorm/blob/master/src/_template/habitat_conditions/06_groundwater_conditions.Rmd)
• old Excel script still available from: http://www.riv.co.za/wv/Scripts.html
• existing script in Watina app

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

[cecileherr]

check this: https://stackoverflow.com/questions/9614433/creating-radar-chart-a-k-a-star-plot-spider-plot-using-ggplot2-in-r (and ggradar)