thisroot.sh not working properly for zsh

The script thisroot.sh is not working in conjunction with the bash-related zsh linux shell. When sourcing thisroot.sh from within the root/bin/ folder, the following error appears:

ERROR: must cd where/root/is before calling . bin/thisroot.sh for this version of bash!

"thisroot.sh" uses the BASH_ARGV env-variable, which is not supported by all bash-related shells. "thisroot.sh" should take care of managing the folder structure, also if sourced from within root/bin/

Please see the attached patch for a suggestion for a fix.

PS: thisroot.csh is handling the source from within the bin/-folder properly.