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/import-csv.h

@@ -26,7 +26,7 @@ extern "C" {
 #endif
 
 int parse_csv_file(const char *filename, struct xml_params *params, const char *csvtemplate, struct divelog *log);
-int try_to_open_csv(struct memblock *mem, enum csv_format type, struct divelog *log);
+int try_to_open_csv(std::string &mem, enum csv_format type, struct divelog *log);
 int parse_txt_file(const char *filename, const char *csv, struct divelog *log);
 
 int parse_seabear_log(const char *filename, struct divelog *log);