Preventted QT slot errors from occuring. Refs #7866.
Tidied up `CatalogDownloadDataFiles`. Refs #7866.

- Removed reference to ISIS.
- Made use of `isAscii` and removed method to `isBinary`.
Added ICAT3 load support. Refs #7866.

- Transformed the string to user-specific ICAT path, in order to directly load it from archives.
Fixed failing test. Refs #7866.

- If filename is invalid or does not exist then we want to return false.