Handling of relative errors

[archaeothommy]

The maintainer meeting on March 30 decided to convert relative errors to absolut errors by default. The user must have the option to convert explicitly to relative errors with being responsible for any unwanted side effects arising from using relative units (see #39 for discussion).

To dos:

• Functions for conversion rel -> abs error and abs -> rel error.
• Include conversion in parser
• Update implementation vignette and documentation of ASTR object as well as in pkgdown YAML