Add user specification of bounds to visus minmax

[cchriste]

Some datasets (e.g., climate simulations) use specific bounds for out-of-range data that are smaller that min/max of a float. visus minmax can not compute usable bounds for these type of datasets. Allow user to specify the valid range so values outside that range can be ignored.

      else if (args[I] == "--min") { min_val = cdouble(args[++I]); }
      else if (args[I] == "--max") { max_val = cdouble(args[++I]); }
      ...