Read-only: It's far too easy to corrupt an HDF5 file. Making it
possible to open a file in read-only mode is an important step
towards making that not quite as easy.

SWMR: As explained in a comment, we can enable it in read-only mode by
default; in read-and-write mode it can be enabled one the constructor
returns, through the function swmrEnable().

As a result of this change, the line

  HDF5::File file(filename, true);

which was legal in the past but not used by anyone to the best of my
knowledge, will no longer compile.