BF: Use space to separate short arg/value when calling CLI tools (e.g. "-i input" instead of "-i=input").

[pauldmccarthy]

Hi @mcraig-ibme, I hope you're well!

We recently had a report on the FSL mailing list (https://www.jiscmail.ac.uk/cgi-bin/wa-jisc.exe?A2=FSL;a059e996.2601) about asl_gui incorrectly calling fsl_anat.

When setting Structural data from to Run FSL_ANAT on structural image, asl_gui constructs the call as:

fsl_anat -i=<t1-image> -o=<output-dir>

where it should be:

fsl_anat -i <t1-image> -o <output-dir>

This results in the following error:

/home/paulmc/fsl/bin/fsl_anat -i=/home/paulmc/fsl_course_data/ASL/T1.nii.gz -o=basil_output/struc
mkdir: invalid option -- 'o'
Try 'mkdir --help' for more information.

Return code: 1

WARNING: command failed

This PR adjusts the command-line construction logic so that an equals sign is only used for flags which start with --, and a space used otherwise.

Thanks!