Use checkmate for argument checking

[athowes]

Have lots of functions which use something like a data.table with certain column names. Need to make sure the inputs to these functions are valid. Currently not doing this. See: https://mllg.github.io/checkmate/

[seabbs]

We also have a specific data.table checker in Epinowcast that @pearsonca is spinning out into coerce_dt. Not sure we need that here though.

[athowes]

Going to close this issue as I have now used checkmate in closed PRs and would be confident about using it in future work.