Improve installation of dependencies

[njtierney]

Provides a new process for installing python dependencies, and a better user experience for dealing with this.

It addresses this by installing the needed python packages into a special "greta-env" conda environment. This can be thought of as similar to a docker container, or a renv.lock file. Our specified python dependencies are all put into this "greta-env" environment, which greta checks exists when loading up greta.

An example of this new process is shown below:

# wipe out the good environment
library(reticulate)
conda_remove("greta-env")

# set up a new, different environment
env <- tempfile("python-env-")
conda_create(env)
use_condaenv(env, required = TRUE)
reticulate::py_discover_config()

# try to run greta
library(greta)
x <- normal(0, 1)
# message appears

Error: We have detected that you do not have the 'greta-env' condaenvironment and python packages setup. You can set these up using:

	install_greta_deps()

and then call:

	library(greta)

in a fresh R session that has not yet initialised Tensorflow.
For more information, see ?install_greta_deps 

# run this, as requested
install_greta_deps()

# <restart here> as requested
library(greta)
x <- normal(0, 1)
# it hangs here for a minute
# I wonder what is taking it so long?
m <- model(x)
draws <- mcmc(m)
# woohoo!

Resolves #396

[njtierney]

Todo list:

• Move installation things from create_greta_env() into install_greta_deps()
• Document potential issues regarding installing miniconda - "If you run reticulate::install_miniconda() when you already have miniconda in the usual location, you end up with two conda installations, which can be awkward" (test this with users at work to see if this makes sense)
• Document any extra things about the greta env somewhere. E.g., advanced users can install their own set up in a different environment if they want! (add a bit of a note about this in the documentation)

[njtierney]

Going to merge this as the only GH action that is breaking is some obscure fail where fields doesn't install