add data directory to imageseries config

[donald-e-boyce]

It would be convenient to specify a data directory in the imageseries config file. As it is, either all the data files are in the current directory or they are in a different directory and every panel has the same directory path before the filename. It would nice to have a single directory specified. For example: this config

image_series:
  format: frame-cache
  data:
    - file: /a/b/c/sample-panel-0.npz
      args: {}
      panel: panel-0  # must match detector key
    - file: /a/b/c/sample-panel-1.npz
      args: {}
      panel: panel-1  # must match detector key

could be replaced with

image_series:
  format: frame-cache
  path:  /a/b/c
  data:
    - file: sample-panel-0.npz
      args: {}
      panel: panel-0  # must match detector key
    - file: sample-panel-1.npz
      args: {}
      panel: panel-1  # must match detector key

See hexrd.config.imageseries