MFix particles reader
We need to add a MFix particles reader to make the this workflow easier.
- add support for opening directory series. AMREX saves plot files which are actually directories. The current workflow doesn't allow for series of directories and folks seem to have to use imaginative approaches to overcome this limitation. Let's add support for it.
- this reader would like to refer to open directories, however the extension matching should match directories that have the files of interest. can we support that?
- implement the reader.