Rely on stable package versions rather than remotes

[athowes]

As a part of the DESCRIPTION we currently rely on GitHub remotes for a number of packages:

Remotes:
    stan-dev/cmdstanr,
    Rdatatable/data.table,
    epinowcast/epinowcast

PR #69 adds brms to this list as there is a bug with the non-HMC inference methods only fixed in the development version of brms (paul-buerkner/brms#1650).

Ideally, we should look to depend on stable package versions. The reasons why are:

When developing R packages, it is generally advisable to avoid depending on GitHub-based dependencies. Instead, using stable versions from CRAN (Comprehensive R Archive Network) is recommended. Here are the reasons why:

1. CRAN checks packages
2. CRAN packages are versioned
3. CRAN provides pre-compiled binaries, making installation easier (faster) for users
4. GH packages require manual compilation (maybe difficult / requires toolchain?)
5. CRAN has an archive of package versions, good for reproducibility
6. GH versions could have breaking changes

I don't know the extent of barriers to doing this. For the brms one it's a matter of time until the fix is on CRAN.

[athowes]

• Remove data.table remote dependency #148: data.table: depend on development version because we were using dt but not using that
• Remove epinowcast package and functions #149: epinowcast: not on CRAN but not using it anymore
• Remove cmdstanr remote dependency #147: cmdstanr: only one needing long term. Use additional repos argument

[athowes]

We use stable package versions now.