minor improvements to OPES_METAD

See #524

thanks to @IsabellGrothaus

This is a harmless mistake, I found it while implementing #653 so I backported the fix

cc: @valsson

This is a harmless mistake, I found it while implementing
#653 so I backported the fix

This is an issue identified while implementing #653

It is not strictly necessary for PLUMED < 2.8, but I backported
it anyway for clarity and to avoid issue when people copy patches
from one version to another

See failures while testing #687

This reverts commit 3c316e1c.