Issue #427: Fix error in set_forecast_unit() that occurs when input is not a data.table

[nikosbosse]

Fixes #427

set_forecast_unit() currently fails when the input is a data.frame() instead of a data.table(). This PR hotfixes the issue by making sure by calling as.data.table() on the input.

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There exists also a more complete fix including tests and more thorough input checks in #437 that have already been reviewed and merged into the development branch. It would have been more ideal to target that PR to main instead of develop, but given that has already happened I think it is best to make a quick fix here. I ran the tests from the already reviewed PR here as well to make sure everything is working as expected, but didn't replicate them to avoid further merge conflicts due to variable names changing between main and dev.
Not ideal, apologies, will target main with future fixes like this one.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4c3bcea) 90.91% compared to head (a1bd8a9) 90.92%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #487   +/-   ##
=======================================
  Coverage   90.91%   90.92%           
=======================================
  Files          23       23           
  Lines        1398     1399    +1     
=======================================
+ Hits         1271     1272    +1     
  Misses        127      127           

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[seabbs]

I'd really rather this had a unit test and merge issues were dealt with but its better to get this fixed than not at all.