add `reinstall_greta_deps()` function

[njtierney]

From discussion with Gerry, it looks like the following helps reset things when greta stops working for whatever reason - it might be useful to wrap this up in a reinstall_greta_deps() function, which does:

remove_greta_env()
remove_miniconda()
install_greta_deps()

Perhaps adding an optional, "restart R now?" option, see #523

[hrlai]

I can confirm that this is necessary for reverting from developmental versions that use TF2 to one that uses TF1.x (e.g., v0.4.2). For example, to revert back to v0.4.2 after doing stuff mentioned in #539 , I had to run the codes above.

[njtierney]

Thanks for this, @hrlai ! Glad to know that this works.