Removed unused local variable. Refs #7637.

- Suppressed unused parameter warning.
Allow rows to be unchecked in datafile table. Refs #7637.
Added load/download validation. Refs #7637.

- Disable load and download button by default, and re-enable when a user has selected at least one
Override QHeadView options to allow checkbox in header. Refs #7637.
Use custom header class to allow checkbox in table header. Refs #7637.
Added select all slot. Refs #7637.
Merge branch 'feature/7637_improve_icat_gui' into develop