Add tests for multi format reader

[lukaspie]

• config file parsing (maybe already (partially) exists)
• ELN parsing (maybe already (partially) exists)
  • /ENTRY vs. /ENTRY[entry] parent_key
  • use of CONVERT_DICT and REPLACE_NESTED
• tests for using different extensions
• custom processing_order
• overwrite_keys
• post_process

Reader functions:

• setup_template
• handle_objects

Individual functions:

• fill_wildcard_data_indices together with `get_data_dims´ of reader
• resolve_special_keys
  • different standard prefixes: @data, @attrs, @data, @link and for simple values
  • a custom prefix
  • a custom function name for a standard prefix (i.e., overloading self.callbacks)
  • single entry, multiple entry
  • linking (see in other readers)
  • Lists as config value: !['@eln', '@attrs', <some-value>]
  • ! (required fields in optional groups) notation for
    • standard prefixes
    • @link
    • !['@eln', '@attrs', '@link']