there is an issue with many xyz file in our regtests

because they do not define a box but the parser was
going on without errors.

now this is fixed but I think we could do it better

Author: carlocamilloni

regtest/crystallization/rt-averaged-q6-spAspB/colvar_aq6.reference

@@ -1,2 +1,2 @@
 #! FIELDS time aq6.mean q6.mean res.bias res.force2
- 0.000000 0.457944 0.511064 2.938250 117.529990
+ 0.000000 0.446223 0.479323 3.066685 122.667398