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>

core/CMakeLists.txt

@@ -41,7 +41,7 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	checkcloudconnection.h
 	cloudstorage.cpp
 	cloudstorage.h
-	cochran.c
+	cochran.cpp
 	cochran.h
 	color.cpp
 	color.h
@@ -51,7 +51,7 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	configuredivecomputerthreads.h
 	connectionlistmodel.cpp
 	connectionlistmodel.h
-	datatrak.c
+	datatrak.cpp
 	datatrak.h
 	deco.c
 	deco.h
@@ -121,9 +121,9 @@ set(SUBSURFACE_CORE_LIB_SRCS
 	import-suunto.cpp
 	import-seac.cpp
 	interpolate.h
-	libdivecomputer.c
+	libdivecomputer.cpp
 	libdivecomputer.h
-	liquivision.c
+	liquivision.cpp
 	load-git.cpp
 	membuffer.cpp
 	membuffer.h