Added regtests for upper and lower walls

regtest/rt9/COLVAR.reference

+#! FIELDS time uwall.bias uwall.force2 lwall.bias lwall.force2
+ 0.000000  0.062  4.935  0.797 127.597
+ 0.050000  0.119  9.505  0.298 47.693
+ 0.100000  0.131 10.488  0.070 11.261
+ 0.150000  0.104  8.287  0.007  1.149
+ 0.200000  0.106  8.463  0.006  0.882

regtest/rt9/Makefile

+../scripts/test.make
\ No newline at end of file

regtest/rt9/config

+type=driver
+# this is to test a different name
+arg="--plumed plumed.dat --stride 10 --timestep 0.005 trajectory.xyz --dump-forces forces --dump-forces-fmt=%10.6f"

regtest/rt9/plumed.dat

+ang1: ANGLE ATOMS=2,6,3
+ang2: ANGLE ATOMS=2,6,7,3
+
+UPPER_WALLS ARG=ang1 AT=0.3 KAPPA=20.0 LABEL=uwall
+LOWER_WALLS ARG=ang2 AT=0.2 KAPPA=40.0 EXP=4 EPS=0.3 OFFSET=0.1 LABEL=lwall
+
+PRINT ...
+  STRIDE=10
+  ARG=uwall.bias,uwall.force2,lwall.bias,lwall.force2 
+  FILE=COLVAR FMT=%6.3f
+... PRINT
+
+ENDPLUMED
+