(v0.6.0) Simplify boundary condition construction

[navidcy]

Description edited by Ashley to reflect discussion below
This PR aims to simplify the boundary condition construction in the simple case of four boundaries that are parallel to lines of constant longitude and latitude, as per the discussion in #174.

To do this, we add a wrapper function that just calls the simple_boundary method for each cardinal direction requested.

This PR includes a breaking API change since rectangular_boundary is renamed to rectangular_boundaries and also some of its args changed.

Closes #174

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[ashjbarnes]

In this PR you're changing the initial condition function to also do the boundary right? I still think that the boundary and initial conditions should be kept as separate functions. If not, then we'll should rename it so that the "initial condition" function doesn't do the boundary right? This is confusing to me

Perhaps we copy - paste the 'iterate over boundaries' file to the setup_rectangular_boundary() function so that this function will default to four automatic boundaries, but user can just provide one if they want

[navidcy]

Oh my bad

[ashjbarnes]

This PR will require a new tag release as it's not backwards compatible. Also need to update the access om2 example with the changes.

[ashjbarnes]

I'm going to approve the PR because it was Navid's originally. Wait on merging until @navidcy is happy and we also fix up the access om2 example to match simultaneously

[navidcy]

I'd like to merge this and then we tag a new release -- how does this sound @ashjbarnes?

[ashjbarnes]

Great! Need to reflect changes in access om2 example