refs #10187. Always close group.

A few issues fixed here. Firstly. I found that the algorithm was failing
when thrown a very large group workspace. This turned out to be because we
needed to close the group within the doExec call. A group i always opened,
so it is necessary to close it.

The second thing done is to fix SaveNexus so that it will pass through
group workspaces intact to SaveNexusProcessed. Under the previous
mechanism, this would not have happened, because the generic Algorithm
group processing code would have exploded out the individual workspaces.
Merge branch 'feature/10187_process_groups' of github.com:mantidproject/mantid into feature/10187_process_groups
refs #10187. Delete existing file if no append.
refs #10187. File var not initialized fix.
refs #10187. stop calling searching for groups following POGO.

Use an optional entry number parameter instead of counting workspace group entries in the file. This was the main bottleneck acording to the previous
POGO report. Seems to have generated a massive speedup. Can save as WG of size 512 in < 8 seconds rather than 138 seconds as previously.
Merge branch 'feature/10187_process_groups' into develop