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>
