Start "output unit management" support

This doesn't actually *do* anything yet, but it introduces the notion of
output units, and allows you to pick metric or imperial.

Of course, since the output doesn't currently care, the units you pick
are irrelevant.  But just wait..

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

dive.h

@@ -126,6 +126,22 @@ struct dive {
 	struct sample sample[];
 };
 
+/*
+ * We keep our internal data in well-specified units, but
+ * the input and output may come in some random format. This
+ * keeps track of those units.
+ */
+struct units {
+	enum { METERS, FEET } length;
+	enum { LITER, CUFT } volume;
+	enum { BAR, PSI, PASCAL } pressure;
+	enum { CELSIUS, FAHRENHEIT, KELVIN } temperature;
+	enum { KG, LBS } weight;
+};
+
+extern const struct units SI_units, IMPERIAL_units;
+extern struct units input_units, output_units;
+
 extern int verbose;
 
 struct dive_table {