cleanup: split out divecomputer functions from dive.c

Since dive.c is so huge, split out divecomputer-related functions
into divecomputer.[c|h], sample.[c|h] and extradata.[c|h].

This does not give huge compile time improvements, since
struct dive contains a struct divecomputer and therefore
dive.h has to include divecomputer.h. However, it make things
distinctly more clear.

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

qt-models/divecomputerextradatamodel.cpp

@@ -1,9 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0
 #include "qt-models/divecomputerextradatamodel.h"
-#include "core/dive.h"
+#include "core/divecomputer.h"
+#include "core/extradata.h"
 #include "core/metrics.h"
 
-
 ExtraDataModel::ExtraDataModel(QObject *parent) : CleanerTableModel(parent)
 {
 	//enum Column {KEY, VALUE};

qt-models/diveplannermodel.cpp

@@ -7,6 +7,7 @@
 #include "qt-models/models.h"
 #include "core/device.h"
 #include "core/qthelper.h"
+#include "core/sample.h"
 #include "core/settings/qPrefDivePlanner.h"
 #include "core/settings/qPrefUnit.h"
 #if !defined(SUBSURFACE_TESTING)