- the development should have been on a dedicated branch instead of on master since the start ...!
- add g++ for cygwin build

- warning was issued on github actions for a build on windows-release with cygwin and gcc v11.2.0

- the argument of `isalpha()` is "int ch" (https://en.cppreference.com/w/c/string/byte/isalpha) but we were passing in a "char" (which may be signed or unsigned depending on platform and compiler)
- similarly to the use in the next line with `toupper()`, the fix is to convert to "int"

AddressSanitizer and LeakSanitizer are not available on cygwin (https://github.com/google/sanitizers, but see https://github.com/google/sanitizers/wiki/AddressSanitizerWindowsPort)

- new step "check_documentation" (only on linux): install doxygen, build SOILWAT2 documentation, and check that no unexpected warnings occurred

- also: re-organized code coverage checks as separate job

- each job needs to check out the repository

- remove "main.yml"
- move job "build_nix" into script "main_nix.yml"
- move job "check_documentation" into script "check_doc.yml"
- move job "build_Windows_Cygwin" into script "main_win.yml"

- update compiler version requirements for unit tests
- update status badges (moving from travis and appveyor to github actions)

- move job "check_documentation" into script "check_doc.yml" (previously in "main.yml")
- install also doxygen-latex and graphviz dependencies

- workflow names are used in badges on README -> long names display poorly

- workflow names are used in badges on README -> long names display poorly

- workflow names are used in badges on README -> long names display poorlygithub actions: use shorter name for doc workflow

…ogy/SOILWAT2 into feature_ghactions_v2

- fix doxygen "warning: documented empty return type ..." for functions that return "void": `lyrSoil_to_lyrTemp()`, `pot_soil_evap_bs()`, `write_headers_to_csv()`

- some doxygen hyperlinks no longer worked correctly -> use mostly markdown-style links (except for creating subpages)
- however, html tags don't work for doxygen documentation (as of v1.9.3) (see issue #299)

- close issue #267 (Remove from doc/doxygen_exceptions.txt: "Unexpected html tag <img>...")
- doxygen issue resolved with v1.8.16 (github actions installs v1.8.17 on ubuntu-release; and I get v1.9.3)