add custom easyblock to install FSL via `conda`

[scratchings]

FSL is now installed via a Conda environment. The default Conda easyblock cannot easily be used because:

• FSL requires that certain environment variables are defined, e.g. FSLOUTPUTTYPE so these would need to be added as a module file extra in the .eb file. As you typically need to avoid overriding these variables if already set by the user this makes the module additions tcl/lmod specific.
• The generic/conda block automatically adds various bin/library folders to system search paths, this is usually NOT what you want to do with FS when installed via conda.
• If the variable FSL_CREATE_WRAPPER_SCRIPTS is set to '1' then wrapper scripts for FSL tools will be created in $root/share/fsl/bin avoiding the pollution of the user's PATH with conda provided bin/libraries.

EB_FSLCONDA creates a clean module file, avoiding the discovery of LD_LIBRARY_PATH/PKG_CONF locations, ensures that the wrapper script directory $root/share/fsl/bin is created and populated, and that the minimum required environment variables are set in the generated module.