Create `Tool` and `Tool_gui` commands on Linux, and expand functionality of `createFSLWrapper`

Historically, FSL GUIs have been named Tool on linux, and Tool_gui on macOS. In FSL ~6.0.7.5, we are now naming them Tool_gui on both linux and macOS for consistency across the platforms. However, to minimise disruption, we will continue creating commands called Tool on linux.

During the transition period some linux GUIs may have both Tool and Tool_gui variants in $FSLDIR/bin/, but others may only have the Tool variant. Therefore the createFSLWrapper script has been updated to create both Tool and Tool_gui wrappers in $FSLDIR/share/fsl/bin/, regardless of whether we only have $FSLDIR/bin/Tool or both $FSLDIR/bin/Tool and $FSLDIR/bin/Tool_gui.

The createFSLWrapper and removeFSLWrapper commands have also been rewritten in Python; createFSLWrapper now has additional functionality, including the ability to specify the source and destination directories, and to create "unresolved" wrapper scripts, which call e.g. ${FSLDIR}/bin/tool rather than /fully/resolved/path/to/fsldir/bin/tool.