add depth lines to hdf5 file

branched off of https://github.com/twdb/hydropick/pull/42 for convenience - could be made standalone if needed

This is basic functionality for reading/writing pick lines to the hdf5 file. It will probably need to be tweaked a little for save functionality.

Also, there's no way to import pick files from the gui. It would be easy to just import all of them when importing a survey but importing single pick files or making the import optional would take a little bit of work.