Use checkmate to check inputs

[Robinlovelace]

This could help generate useful error messages and reduce the chances of the package outputting incorrect results due to incorrect input data. See https://github.com/mllg/checkmate for details.

I see this as a nice to have rather than essential and time may be better spent on creating a formal class system for OD data as per itsleeds/od#20

[natesheehan]

+1 this would still be good to add in. When I get some time I am happy to look at this, maybe after the next release.

[Robinlovelace]

I don't think it's a priority but yes some defensive programming to identify dodgy inputs early could be good.