"Working on a cluster" -- assumption of user log-in state

[reid-a]

The "Working on a cluster" episode begins with the assumption that the user is logged in to the head node of a cluster ("what computer have we logged into?"), but the actual step of logging in to the head node is not called out, either at the start of this lesson, or the end of the previous one. Since the point of the description is to clarify that there are multiple contexts or scopes within a cluster, it's important to make this step explicit.

[psteinb]

Very well spotted. Please submit a PR to add this as a tiny paragraph. ssh/scp should be included in hpc-shell IIRC.