Add time-to-surface sample type

This adds the ability to report time to surface (TTS) as a sample, which
a number of backends will want.

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

include/libdivecomputer/parser.h

@@ -46,9 +46,13 @@ typedef enum dc_sample_type_t {
 	DC_SAMPLE_PPO2,
 	DC_SAMPLE_CNS,
 	DC_SAMPLE_DECO,
-	DC_SAMPLE_GASMIX
+	DC_SAMPLE_GASMIX,
+	DC_SAMPLE_TTS,		// time to surface in seconds
 } dc_sample_type_t;
 
+// Make it easy to test support compile-time with "#ifdef DC_SAMPLE_TTS"
+#define DC_SAMPLE_TTS DC_SAMPLE_TTS
+
 typedef enum dc_field_type_t {
 	DC_FIELD_DIVETIME,
 	DC_FIELD_MAXDEPTH,