Update maximum-precision float format to .17g

See http://stackoverflow.com/questions/16839658 -- although
in most cases 16 decimal digits suffices for a double
precision value, in a handful of instances 17 are necessary.
At the command line, see the difference between:

$ printf %.17g 1.0000000000000001

and

$ printf %.16g 1.0000000000000001

Since SPINS does not provide bit-for-bit compatibility when
restarting (because of the startup timestepping approach),
this change is unlikely to make any significant difference
to existing or future cases.

src/BaseCase.cpp

@@ -344,7 +344,7 @@ void BaseCase::check_and_dump(double clock_time, double real_start_time,
             FILE * dump_file; 
             dump_file = fopen("dump_time.txt","w");
             assert(dump_file);
-            fprintf(dump_file,"The dump time was:\n%.16g\n", sim_time);
+            fprintf(dump_file,"The dump time was:\n%.17g\n", sim_time);
             fprintf(dump_file,"The dump index was:\n%d\n", plot_number);
             fclose(dump_file);
         }