Allow lazy loading from app5 files

[argerlt]

As discussed in #464, lazy loading of .app5 files would be a desirable additional feature.

For the SPED datasets, the offsets calculated for use in np.frombuffer should be sufficient for loading data from disk. A similar process could also be employed for STEM datasets.

If this feature was added, it would also make sense to depreciate dryrun, as lazy_loader would do almost the same thing in only a slightly longer compute time.