Reduce NAMESPACE of conversion functions

[joshwlambert]

Currently there are a few conversion functions in {epiparameter}:

• convert_gamma_params
• convert_gamma_summary_stats
• convert_geom_params
• convert_geom_summary_stats
• convert_lnorm_params
• convert_lnorm_summary_stats
• convert_nbinom_params
• convert_nbinom_summary_stats
• convert_weibull_params
• convert_weibull_summary_stats
• gamma_meansd2shapescale
• gamma_shapescale2meansd
• geom_mean2prob
• geom_prob2mean
• lnorm_meanlogsdlog2meansd
• lnorm_meansd2meanlogsdlog
• nbinom_meandisp2probdisp
• nbinom_probdisp2meandisp
• weibull_meansd2shapescale
• weibull_shapescale2meansd

It was raised in #151 that it would be better to reduce the number of conversion functions and instead have a few general functions that encapsulate this functionality. This will reduce the API of the package, be easier to maintain and scale better if we want to add more conversions in the future.

[joshwlambert]

When this issue was raised in #151 it was mentioned that the function could be dispatched based on the input with the example:

epiparam() |> 
  mutate(params = convert_summary_stats(prob_distribution, mean, sd))

Therefore, it would be good to check if these conversion functions could be turned into S3 generics and dispatched on <epiparam> and <epidist> objects as well as using the default character string implementation that already works.

[joshwlambert]

PR #165 implements the major change requested in this issue, therefore, I will merge this PR before making the change to implement convert_params() and convert_summary_stats() as S3 generics, and make a new issue for this.