looper config should allow for pipestat pephub path.

[donaldcampbelljr]

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Currently, working on the docs, and I realized that looper needs to be able to add the pephub_path key to the pipestat namespace so that this path can be passed along to pipestat via the command template (either directly or via the generated config_file).

Also, pipestat currently only receives the pephub_path as an argument. It cannot obtain this from the config file. Therefore, pipestat should also be adjusted to read the pephub_path from the config file.