Break up lesson episodes into smaller chunks

[jcohen02]

From discussion with @aturner-epcc and @sstevens2, it was decided that it would be good to reduce the size of the episiodes in this lesson to make things a little more modular and, in the future, provide more options to mix and match optional pieces of content.

When this material was first run, each episode took a little under 1 hour, the aim is to break the material down, probably into 7-8 episodes, each of which would contain about 25-35 minutes worth of material.

[aturner-epcc]

For episode 2 (Working with Singularity containers), potentially split into:

2.1 Image cache and storage

2.2 Using Docker images with containers

2.3 Accessing files in Singularity containers

2.4 Running commands and/or accessing shell in containers

[aturner-epcc]

Split of episode 2 is complete, see: 4d75af5

[jcohen02]

Part II of the material (building containers, running MPI jobs) was split into three episodes in cd99f89 and the first part of the material has now been split into 5 episodes so we have a total of 8 episodes as suggested originally in this issue.

@aturner-epcc, are you happy to close this now or do you feel this can be restructured further?