core: convert sample.c to C++ and add default constructor

This changes default behavior when creating a sample struct
in C++ code: it is now initialized to default values. If this
ever turns out to be a performance problem, we can either add
additional constructors or use special functions that do
not initialize memory, such as make_unique_for_overwrite.

This removes non-standard (respectively >C++20) constructs,
namely designated initializers.

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

core/sample.h

@@ -33,6 +33,9 @@ struct sample                         // BASE TYPE BYTES  UNITS    RANGE
 	bool in_deco;                     // bool       1    y/n      y/n                  this sample is part of deco
 	bool manually_entered;            // bool       1    y/n      y/n                  this sample was entered by the user,
 					  //                                               not calculated when planning a dive
+#ifdef __cplusplus
+	sample();			  // Default constructor
+#endif
 };	                                  // Total size of structure: 63 bytes, excluding padding at end
 
 extern void add_sample_pressure(struct sample *sample, int sensor, int mbar);