add volume rounding prec stub to elements

isotest/bits/elems.v

@@ -59,6 +59,9 @@ mut:
 	name string
 	elem_type ElemType
 	density int // kg/m3 // XXX per bit?
+	// XXX TODO: rounding precision: different precision
+	// for different elements (more crude means less precision for volume calculations)
+	prec int
 	flameable bool
 	liquid bool // XXX is technically a liquid in its normal state?
 
@@ -100,6 +103,7 @@ fn new_elem() &Elem {
 	e := &Elem{
 		solid_state: .solid,
 		aggregate_state: .solid,
+		//prec: 4, // XXX what is default precision?
 	}
 	
 	return e