Updates to 24-keras-classify.md ("Classifying Smartphone Apps with Keras")

This is an ongoing work by Wirawan to update the "Classifying Smartphone Apps with Keras" episode

Proposed improvements

1. Move the convergence issue from [30-model-tuning.md] to here.

2. Model saving & reloading

3. Demo inference

4. Continuing training (from checkpoint)

5. Saving metadata (currently in [``]) . See https://deap-secure-lesson-04-nn-dparr-005-c792a2.gitlab.io/31-batch-tuning-hpc/index.html , under heading "Metadata".

In the subsequent replies, let's discuss each of these for/against placing them in this episode.