Fixed typo "recomment" -> "recommend"

Can be used for numeric comparisons in precompiler directives

Error is this one:
../maketools/makelinks.sh: line 24: echo: write error: Broken pipe

It's triggered by the comparison with a <(echo) file, since the comparison
might fail before echo completes. If this works I will backport it

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