Scanning via `mcrun -N` can fail due to `STDOUT` capture

[g5t]

Problem

When run in 'scan' mode, mcrun captures STDOUT and STDERR in order to identify detector lines. https://github.com/McStasMcXtrace/McCode/blob/c725e6fcd621f1103b37b5d46c913be60df8dc1a/tools/Python/mcrun/optimisation.py#L261

For not-entirely-clear reasons, in some cases the scan can hang indefinitely -- possibly due to overflowing the buffer as seems to be indicated in the Popen.communicate documentation

Solution

This problem is fixed by switching from STDOUT parsing to reading the requisite information back from each scan-point mccode.sim file, and not capturing either STDOUT or STDERR.

Side effects

If change is implemented

Scan progress indication

Output from each simulation step in the scan will be sent to STDOUT allowing a user to easily identify that a long-running process is progressing instead of wondering why no output is produced, in contrast to the single-point (no -N) mode of mcrun.

If no change is implemented

Possible undefined behavior

In its current state, a user could inadvertently break mcrun in scan mode by writing a component that produces lines which match the regular expression used to identify detectors https://github.com/McStasMcXtrace/McCode/blob/c725e6fcd621f1103b37b5d46c913be60df8dc1a/tools/Python/mcrun/mccode.py#L379