Generalize queries to work if there is more than just 1 container in a pod

[rptaylor]

Some queries currently only work for the case of a pod with one container.

At least for some queries, such as self.cores https://github.com/rptaylor/kapel/blob/master/python/KAPEL.py#L58 , this is easy to add by just putting sum by (pod) in front.

Edit: perhaps my initial assessment was mistaken, in any case for some queries it is not that easy: #76 (comment)

Reference discussion: #54 (comment)

[rptaylor]

This could also be addressed by #71 , using kube_pod_resource_request instead of container-level resources.