Add input validation

[Bisaloo]

I think many errors (both in my own usage and in some bug reports such as #267) seem to come from invalid input.

I understand that what constitutes a valid input is documented but I believe some extra hand-holding by failing early with clear error messages on invalid input would go a long way in helping users.

A main topic to discuss is probably what would be the best framework to do this input checking. Some options are presented in the 'Checking the inputs of your R functions' blog post.

[sbfnk]

One thing perhaps to do is for every of the ..._opts functions in https://github.com/epiforecasts/EpiNow2/blob/main/R/opts.R that are used to passed options to estimate_infections in

EpiNow2/R/estimate_infections.R

Line 220 in dd484e2

estimate_infections <- function(reported_cases,

to return their own S3 class. We could then check in estimate_infections that the arguments are of that class, avoiding instances where this is confused as e.g. in

EpiNow2/tests/testthat/test-epinow.R

Line 101 in dd484e2

stan = delay_opts(

.

[jamesmbaazam]

@seabbs I would like to take a stab at this. Please assign me.

[seabbs]

Awesome - looking forward to seeing what you come up with. Make sure to target the develop branch with your changes vs main.

[sbfnk]

There is no more develop branch so you can target main directly when taking this on.

[jamesmbaazam]

Noted.