Added regtest for mf_pdb

I put no box info in the first frame so as to check this:
f4ca4c5d.

regtest/basic/rt-molfile-6/Makefile

+include ../../scripts/test.make

regtest/basic/rt-molfile-6/config

+type=driver
+arg="--plumed plumed.dat --mf_pdb input.pdb"

regtest/basic/rt-molfile-6/input.pdb

+REMARK    GENERATED BY TRJCONV
+TITLE     Protein in water t=   0.00000
+REMARK    THIS IS A SIMULATION BOX
+MODEL        1
+ATOM      1  N   SER   189      64.220 109.290  50.970  1.00  0.00
+ATOM      2  H1  SER   189      64.120 110.210  51.360  1.00  0.00
+TER
+ENDMDL
+REMARK    GENERATED BY TRJCONV
+TITLE     Protein in water t=   0.00000
+REMARK    THIS IS A SIMULATION BOX
+CRYST1  112.765  112.765  112.765  60.00  60.00  90.00 P 1           1
+MODEL        1
+ATOM      1  N   SER   189      64.220 109.290  50.970  1.00  0.00            
+ATOM      2  H1  SER   189      64.120 110.210  51.360  1.00  0.00            
+TER
+ENDMDL
+REMARK    GENERATED BY TRJCONV
+TITLE     Protein in water t=   0.00000
+REMARK    THIS IS A SIMULATION BOX
+CRYST1   12.765   12.765   12.765  60.00  60.00  90.00 P 1           1
+MODEL        1
+ATOM      1  N   SER   189      64.220 109.290  50.970  1.00  0.00
+ATOM      2  H1  SER   189      64.120 110.210  51.360  1.00  0.00
+TER
+ENDMDL
+REMARK    GENERATED BY TRJCONV
+TITLE     Protein in water t=   0.00000
+REMARK    THIS IS A SIMULATION BOX
+MODEL        1
+ATOM      1  N   SER   189      64.220 109.290  50.970  1.00  0.00
+ATOM      2  H1  SER   189      64.120 110.210  51.360  1.00  0.00
+TER
+ENDMDL
+

regtest/basic/rt-molfile-6/out.gro.reference

+Made with PLUMED t=0.000000
+2
+    0         X    1   6.422  10.929   5.097
+    0         X    2   6.412  11.021   5.136
+   0.0000000    0.0000000    0.0000000    0.0000000    0.0000000    0.0000000    0.0000000    0.0000000    0.0000000
+Made with PLUMED t=1.000000
+2
+    0         X    1   6.422  10.929   5.097
+    0         X    2   6.412  11.021   5.136
+  11.2764999   11.2764999    7.9736896    0.0000000    0.0000000    0.0000000    0.0000000    5.6382500    5.6382501
+Made with PLUMED t=2.000000
+2
+    0         X    1   6.422  10.929   5.097
+    0         X    2   6.412  11.021   5.136
+   1.2765000    1.2765000    0.9026218    0.0000000    0.0000000    0.0000000    0.0000000    0.6382500    0.6382500
+Made with PLUMED t=3.000000
+2
+    0         X    1   6.422  10.929   5.097
+    0         X    2   6.412  11.021   5.136
+   1.2765000    1.2765000    0.9026218    0.0000000    0.0000000    0.0000000    0.0000000    0.6382500    0.6382500

regtest/basic/rt-molfile-6/plumed.dat

+DUMPATOMS FILE=out.gro ATOMS=1-2