Improve file creation in dump_equal_chunks

[Markus-Goetz]

Right now dump_equal_chunks ALWAYS creates a file with the access mode H5_ACC_TRUNC (overwrite if exists). That is a problem as I cannot do the following

Dataset labels1("foobar.h5", "labels1"); Dataset labels2("foobar.h5", "labels2"); labels1.dump_equal_chunks(); labels2.dump_equal_chunks();

The updates of the labels1 will be lost as labels2 will recreate the file foobar.h5 and by that erase all previous updates. The method should either:

• Check whether the file already exists and if not create it, or
• Always try to create it with H5_ACC_EXCL and afterwards open if with H5F_open if needed