WOMBAT-lite documentation

[pearseb]

Initial draft of docs

[dougiesquire]

This is terrific @pearseb - really well done.

The references to the variable names are very helpful. There are a few instances where they are missing and others where they are missing. I've suggested adding a few, but probably missed some.

[pearseb]

Thanks Dougie will make those corrections now and will recommit.

[pearseb]

I've made some major additions and clean-up to the WOMBAT-lite documentation. These include:

• more consistent labelling of units and "in-code" variable names
• more literature citations and justifications of the equations
• the addition of a "update_from_bottom" page to describe for the permanent burial of detritus and calcium carbonate

[dougiesquire]

I've now included the commits from #91 as we discussed.

I'll now interactively rebase this into three commits:

• One for the WOMBATlite code changes
• One for the WOMBATlite docs
• One for the WOMBATmid bug fix

[dougiesquire]

I've also removed the now-unused fcolloid parameter and made some formatting changes for fortitude. I've squashed these into the "WOMBATlite: major update including:" commit

[dougiesquire]

• One for the WOMBATmid bug fix

I ended up dropping this commit (6314aef) because it was declaring a variable this isn't actually used yet.

I think this commit was supposed to go into the wombatmid-NOSC branch so I've added it there.