stack trace is fully parsed only when accessed with stack()
or when PLUMED_STACK_TRACE=yes

Make sure incorrect conversions trigger an exception

I tested by changing --print-prog-name to --print-prog-namex (not existing) so as to reproduce
the bug, and this seems to fix it so I merge it.

This reverts commit 44c00a9d.

See #727

This reverts commit 922f11d2.

See #727

This reverts commit 412ac970.

See #727

Fix #726

This requires fixing a regtest

Should have gone with 702cea4b

See #726

GHBFIX-Allowing custom fields in input files, adding input files for …

Because of possibilities of unexpected behavior, firstly only the correction attempt for the manual.