Template should export a minimal set of greta functions

[njtierney]

For example, functions like:

• check_tf()
• as.greta_array()
• distrib()
• check_dims()
• op()
• fl()

[njtierney]

Talking to @goldingn about this, one path might be to create a greta.internals package.

This could contain:

• The testing code used inside of greta inside of tests/testthat/helpers.R, including all of the extra distributions etc
• All of the .internals from greta

This has a drawback of creating more dependencies, for example, some of the extra distribution packages like: extraDistr, fields, MCMCpack, mvtnorm, truncdist.

Decoupling these internals from greta might end up creating more work, but it might also help improve testing, distribution of key greta code to other packages, and help simplify certain code designs. Something to think about! Also see greta-dev/greta.distributions#12 for a related topic