Improve consistency of NA handling #165
Description
Whilst the error and warning identified in #111 is fixed, there is still a need to improve the consistency of NA handling throughout the code base. One option would be to remove all NAs up front and put them back at the end. This needs some thinking through, as there may be particular instances where it is not appropriate to simply ignore the NAs - although decisions relating to how to handle this are probably out of the scope of the automation provided by this package. We simply need to provide a clear approach that gives users sufficient control over NA handling.