core: turn a memblock in the parser to std::string

This avoid memory-management troubles. Had to convert a few
of the parsers (cochran, datatrak, liquivision) to C++.
Also had to convert libdivecomputer.c. This was less
painful than expected.

std::string is used because parts of the code assumes
that the data is null terminated after the last character
of the data. std::string does precisely that.

One disadvantage is that std::string clears its memory
when resizing / initializing. Thus we read the file onto
freshly cleared data, which some might thing is a
performance regression. Until someone shows me that this
matters, I don't care.

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

tests/testparse.cpp

@@ -237,10 +237,10 @@ void TestParse::testParseNewFormat()
 
 void TestParse::testParseDLD()
 {
-	struct memblock mem;
 	QString filename = SUBSURFACE_TEST_DATA "/dives/TestDiveDivelogsDE.DLD";
 
-	QVERIFY(readfile(filename.toLatin1().data(), &mem) > 0);
+	auto [mem, err] = readfile(filename.toLatin1().data());
+	QVERIFY(err > 0);
 	QVERIFY(try_to_open_zip(filename.toLatin1().data(), &divelog) > 0);
 
 	fprintf(stderr, "number of dives from DLD: %d \n", divelog.dives->nr);