core: default initialize units-type objects to 0

Makes the code much nicer to read.

Default initialize cylinder_t to the empty cylinder.

This produces lots of warnings, because most structure are now
not PODs anymore and shouldn't be erased using memset().

These memset()s will be removed one-by-one and replaced by
proper constructors.

The whole ordeal made it necessary to add a constructor to
struct event. To simplify things the whole optimization of
the variable-size event names was removed. In upcoming commits
this will be replaced by std::string anyway.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/divesite.h

@@ -14,7 +14,7 @@ struct dive_site
 	uint32_t uuid = 0;
 	std::string name;
 	std::vector<dive *> dives;
-	location_t location = { { 9 }, { 0 } };
+	location_t location;
 	std::string description;
 	std::string notes;
 	taxonomy_data taxonomy;