Add ability to read dark data from HDF5 file

[psavery]

In the simple image series loader, sometimes the dark data is in the same HDF5 file as the image data, and sometimes it is in a separate HDF5 file. We have an example from APS of this, and CHESS mentioned they will have some examples of this as well with their new detectors.

We should add the ability to load dark data from an HDF5 file.

We should potentially also add some logic for automatically detecting the "type" of HDF5 file, if we can, and automatically selecting the dark data within the HDF5 file if the type was detected. This will simplify usage so that if the image data is loaded from one of these HDF5 files, the dark field will be populated automatically.