refs #8124. TDD config type.

Config class can be used to provide a consumable and immutable reference to an xml config file. Tests provided.
refs #8124. ParseError is version dependent

ParseError seems to be version dependent. Better not to specify the exception type.
refs #8124. Fix path with default constructor.

Need to assume that the config.xml is in the same directory as the config.py module.
refs #8124. Make changes post code review.
refs #8124. Remove supplied config file.
Merge remote-tracking branch 'origin/feature/8124_reflectometry_config'