Unify read(T*) and write_raw(T*).

[1uc]

It's convenient to have the option of telling HighFive to just write from a pointer. Currently, one does this with:

dset.read(ptr);
dset.write_raw(ptr);

We should unify the two. Potentially, important twist to consider are things like std::string* which points to a pre-allocated array of strings. Which can't be used in *_raw because it'll segfault. However, it would make sense for read(ptr) and write(ptr) to use the pre-allocated array of std::strings and then copy the strings into/from the pre-allocated array from/to an internal buffer used to read/write the strings.

[1uc]

Please consult the migration guide, it might help modernizing code.