hdf5: basic support for reading metadata
hdf5: allow to read back wavelength

also add the support for MD_WL_LIST: another form for the wavelengths when a polynomial cannot be used.
gui: connect the "Select image" button to actually open a file

works only with HDF5 images from sparc for now.
gui: fix format of spectrum stream from acquisition

make sure it's 5D to indicate the last dimension is C
gui acquisition: ensure that the name of the stream is recorded separately per data