ZSH support (#1593)
ZSH support (#1593)

* sct_env: support zsh too (#1590)

Thankfully the syntax is not too different.

* sct_env: realpath not available on OSX (#1590)

* sct_env: fixup how to get current script path for zsh

https://unix.stackexchange.com/a/351658

Former-commit-id: 7e643308a78ae568fe8af039c18112802f56ea7f
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox