Implement a regex clean-up feature #1
Description
The config file should allow fields to be "cleaned up" by applying a change rule defined by a regular expression evaluated after mapping a file ends.. Many mapping tickets ask for filtering specific values. While this is not possible in the general case (since e.g. "null" also appears in "annullment" and "the null hypothesis"), it could be resolved nicely in an element-specific fashion using this feature.