core: add CRTP base class to unit types

The goal here is to add general addition and scalar multiplication
functions to the unit types.

Thereto, we need a CRTP
(https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern)
base class.

However, this breaks compound initialization, so we have to use
named initializers:
	weight_t { 2000 } -> weight_t { .grams = 2000 }
The good thing is that this is exactly how these classes were
supposed to be used: make the unit explicit!

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

qt-models/cylindermodel.cpp

@@ -86,7 +86,7 @@ static QVariant gas_usage_tooltip(const cylinder_t *cyl)
 	volume_t start = cyl->gas_volume(startp);
 	volume_t end = cyl->gas_volume(endp);
 	// TOOO: implement comparison and subtraction on units.h types.
-	volume_t used = (end.mliter && start.mliter > end.mliter) ? volume_t { start.mliter - end.mliter } : volume_t();
+	volume_t used = (end.mliter && start.mliter > end.mliter) ? volume_t { .mliter = start.mliter - end.mliter } : volume_t();
 
 	if (!used.mliter)
 		return gas_wp_tooltip(cyl);

qt-models/divelocationmodel.cpp

@@ -303,7 +303,6 @@ bool GPSLocationInformationModel::filterAcceptsRow(int sourceRow, const QModelIn
 
 GPSLocationInformationModel::GPSLocationInformationModel(QObject *parent) : QSortFilterProxyModel(parent),
 	ignoreDs(nullptr),
-	location({{0},{0}}),
 	distance(0)
 {
 	setSourceModel(LocationInformationModel::instance());

qt-models/divepicturemodel.cpp

@@ -15,8 +15,7 @@
 
 PictureEntry::PictureEntry(dive *dIn, const picture &p) : d(dIn),
 	filename(p.filename),
-	offsetSeconds(p.offset.seconds),
-	length({ 0 })
+	offsetSeconds(p.offset.seconds)
 {
 }