updated and slightly refactored tabulation for vashishta pair style

- tables are now dimensioned by nelements instead of ntypes
- tables are only created if used
- correctly identify max size of table
- add test for illegal cutoff for tabulation
- allocated memory for tables is accounted for
- add example input using 16-bit tables

examples/vashishta/in.sio2-tabulated

@@ -0,0 +1,29 @@
+# test Vashishta potential for quartz
+
+units		metal
+boundary	p p p
+
+atom_style	atomic
+
+read_data	data.quartz
+
+replicate       4 4 4
+velocity	all create 2000.0 277387 mom yes
+displace_atoms	all move 0.05 0.9 0.4 units box
+
+pair_style 	vashishta
+pair_coeff	* *  SiO.1990.vashishta Si O
+pair_modify	table 16
+
+neighbor	0.3 bin
+neigh_modify	delay 10
+
+fix		1 all nve
+thermo		10
+timestep	0.001
+
+#dump		1 all cfg 10 *.cfg mass type xs ys zs vx vy vz fx fy fz
+#dump_modify	1 element Si O
+
+run		100
+