Test READ EVERY with atoms

See #619

regtest/basic/rt19/CC2.reference

+#! FIELDS time r2
+ 0.000000   1.4960
+ 0.050000   1.5030
+ 0.100000   1.4960
+ 0.150000   1.5020
+ 0.200000   1.5020

regtest/basic/rt19/input_colvar2

+#! FIELDS time rmsd0 rmsds0 rmsd1 rmsds1 rmsd2 rmsds2 rmsd3 rmsds3 rmsd4 rmsds4 sum @11.bias @11.force2
+ 0.000000  1.496  1.643  1.496  1.643  1.337  1.486  1.336  1.486  2.329  2.130 16.381  6.550 13.100
+ 0.050000  1.502  1.650  1.503  1.650  1.347  1.497  1.347  1.496  2.332  2.128 16.452  6.294 12.589
+ 0.100000  1.503  1.653  1.504  1.653  1.351  1.502  1.350  1.501  2.324  2.120 16.461  6.262 12.525
+ 0.150000  1.502  1.653  1.502  1.654  1.350  1.503  1.350  1.503  2.308  2.108 16.433  6.361 12.721
+ 0.200000  1.496  1.649  1.496  1.649  1.347  1.500  1.346  1.499  2.278  2.080 16.340  6.696 13.392
+ 0.000000  1.496  1.643  1.496  1.643  1.337  1.486  1.336  1.486  2.329  2.130 16.381  6.550 13.100
+ 0.050000  1.502  1.650  1.503  1.650  1.347  1.497  1.347  1.496  2.332  2.128 16.452  6.294 12.589
+ 0.100000  1.503  1.653  1.504  1.653  1.351  1.502  1.350  1.501  2.324  2.120 16.461  6.262 12.525
+ 0.150000  1.502  1.653  1.502  1.654  1.350  1.503  1.350  1.503  2.308  2.108 16.433  6.361 12.721
+ 0.200000  1.496  1.649  1.496  1.649  1.347  1.500  1.346  1.499  2.278  2.080 16.340  6.696 13.392

regtest/basic/rt19/plumed.dat

@@ -22,6 +22,9 @@ PRINT ARG=* FILE=COLVAR.nores.gz FMT=%6.3f RESTART=NO
 r1: READ VALUES=rmsd0  FILE=input_colvar.gz
 PRINT ARG=r1 STRIDE=10 FILE=CC.gz FMT=%8.4f
 
+r2: READ VALUES=rmsd0  FILE=input_colvar2 EVERY=2 IGNORE_TIME
+PRINT ARG=r2 FILE=CC2 FMT=%8.4f
+
 METAD FILE=HH.gz ARG=rmsd0 SIGMA=1 HEIGHT=0.1 PACE=10 FMT=%10.6f
 
 ENDPLUMED