Automatically install packages when requested

[richfitz]

Small PR which means that users can just install hermod and we'll grab the rest as we need them.

We need to (later, I think) work out how to flag when version numbers are incompatible, that might take some care.

If you want to see this in action:

remove.packages(c("hermod", "hermod.windows", "conan2"))

then from somewhere on the network share:

> hermod::hermod_init(".")
✔ Initialised hermod at '.'
ℹ Next, call 'hermod_configure()'
> hermod::hermod_configure("windows")
ℹ Trying to install 'hermod.windows'
ℹ To prevent this, set options(hermod.no_install_missing = TRUE)
Installing package into ‘/home/rfitzjoh/lib/R/library’
(as ‘lib’ is unspecified)
also installing the dependency ‘conan2’

trying URL 'https://mrc-ide.r-universe.dev/src/contrib/conan2_1.9.90.tar.gz'
Content type 'application/x-gzip' length 1139673 bytes (1.1 MB)
==================================================
downloaded 1.1 MB

trying URL 'https://mrc-ide.r-universe.dev/src/contrib/hermod.windows_0.1.0.tar.gz'
Content type 'application/x-gzip' length 110784 bytes (108 KB)
==================================================
downloaded 108 KB

* installing *source* package ‘conan2’ ...
** using staged installation
** R
** inst
** byte-compile and prepare package for lazy loading
** help
*** installing help indices
*** copying figures
** building package indices
** testing if installed package can be loaded from temporary location
** testing if installed package can be loaded from final location
** testing if installed package keeps a record of temporary installation path
* DONE (conan2)
* installing *source* package ‘hermod.windows’ ...
** using staged installation
** R
** inst
** byte-compile and prepare package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded from temporary location
** testing if installed package can be loaded from final location
** testing if installed package keeps a record of temporary installation path
* DONE (hermod.windows)

The downloaded source packages are in
	‘/tmp/RtmpJAdNQJ/downloaded_packages’
✔ Installation of 'hermod.windows' successful

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0e52b7c) 100.00% compared to head (722c1ac) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #19   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          357       380   +23     
=========================================
+ Hits           357       380   +23     

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[weshinsley]

I think that all looks good - I've suggested changes from
hermod.no_install_missing to hermond.auto_install_missing - although there were a few changes - perhaps I should have done that in a non-web commit...