core: add constructor/destructor pairs to dive and divecomputer

This allows us to use non-C member variables. Convert a number
of pointers to unique_ptr<>s.

Code in uemis-downloader.cpp had to be refactored, because
it mixed owning and non-owning pointers. Mad.

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

core/parse.h

@@ -54,7 +54,7 @@ struct parser_state {
 	enum import_source import_source = UNKNOWN;
 
 	struct divecomputer *cur_dc = nullptr;			/* non-owning */
-	struct dive *cur_dive = nullptr;			/* owning */
+	std::unique_ptr<dive> cur_dive;				/* owning */
 	std::unique_ptr<dive_site> cur_dive_site;		/* owning */
 	location_t cur_location;
 	struct dive_trip *cur_trip = nullptr;			/* owning */