Save/Restore to HDF5

[newville]

need save/restore functionality:

save_group(filename, *args)

to save groups or data listed to an HDF5 file, with some Larch-specific metadata to signal how to read it back, and a corresponding

data = restore_group(filename, itemname) 

Possibly include an optional:

restore_group(filename, all=True) 

to restore all groups to their saved names in the current session.

Unlike hdf5_group(), this should open, read, copy data into session, and close the save file.

[newville]

seems to be work, but needs more exercising and testing. Especially needed are params / fits for bothe minimize() and feffit().

[maurov]

Related to #363