curves_concentration_max and ec error

description

• ec() raises an error if the requested curve is too short to calculate the result.
• ecxsys() gains a new argument curves_concentration_max which sets the maximum concentration for the curves.
• Improve merge request template.
• Update readme example.

system details

• RStudio Version: 1.2.5033
• R Version: 3.6.2
• OS Version: Windows 10 x64 (build 17763)

checklist

• This merge introduces new features.
  • new features are documented
  • new features have tests
• update documentation including examples (if necessary)
• update and polish NEWS.md
• update version in DESCRIPTION
• update date in DESCRIPTION
• no remaining TODO, FIXME, or debug prints anywhere in the source files
• devtools::document()
• devtools::check() without errors or warnings