Add convenience function set_libraries!

[JoshuaLampert]

This adds a function set_libraries! to conveniently set the preferences for a custom HDF5 library.

closes #1114

[JoshuaLampert]

Do we (alternatively? additionally?) want to allow the user to pass one path to a directory, where both libraries are located?

[JoshuaLampert]

How can I set the docstring without the documenter complaining?

[mkitti]

You should add set_libraries! to the list of documented functions.

[JoshuaLampert]

Thanks @mkitti! I also removed not_low_level_api, which appears to be unused and was quite confusing to me.

[mkitti]

Do we (alternatively? additionally?) want to allow the user to pass one path to a directory, where both libraries are located?

I'm still thinking about this. It seems to mainly add complexity