Added regtest on DRMSD

I suspect a problem in v2.1 with DRMSD cutoff.

This test is working properly in v2.0

regtest/basic/rt34/drmsdx.reference

+#! FIELDS time drmsdx1 drmsdx2
+ 0.000000 1.121735 1.666023
+ 0.050000 1.141275 1.646233
+ 0.100000 1.120122 1.627603
+ 0.150000 1.101672 1.626133
+ 0.200000 1.077282 1.633305

regtest/basic/rt34/plumed.dat

@@ -4,6 +4,10 @@ drmsdn:       DRMSD REFERENCE=test.pdb LOWER_CUTOFF=0.0 UPPER_CUTOFF=15.0 NUMERI
 drmsdnopbc:   DRMSD REFERENCE=test.pdb LOWER_CUTOFF=0.0 UPPER_CUTOFF=15.0 NOPBC
 drmsdnopbcn:  DRMSD REFERENCE=test.pdb LOWER_CUTOFF=0.0 UPPER_CUTOFF=15.0 NOPBC NUMERICAL_DERIVATIVES
 
+# additional check with short cutoff
+drmsdx1:       DRMSD REFERENCE=test.pdb LOWER_CUTOFF=0.0 UPPER_CUTOFF=0.1
+drmsdx2:       DRMSD REFERENCE=test.pdb LOWER_CUTOFF=0.0 UPPER_CUTOFF=0.2
+PRINT ARG=drmsdx1,drmsdx2 FILE=drmsdx
 
 PRINT ...
   STRIDE=10