Use common matplotlib style sheet in all ocean plots

[xylar]

This merge:

• makes a shared use_mplstyle() function for loading and using the matplotlib style file
• moves horizontal visualization to its own module to avoid circular imports
• adds the 3 public visualization functions to the main polaris.viz file for easier importing.

It also calls use_mplstyle() for all other plots in the ocean component.

Checklist

• [x] Developer's Guide has been updated
• [x] API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• [x] Documentation has been built locally and changes look as expected
• [x] Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I ran most tests (every ocean test except the higher res RPE tests). I looked through all the resulting images and they look reasonable. Notably, the cosine bell convergence plots look similar, whereas they had different font sizes in earlier testing in #126 .

icos: qu:

Ideal age plots look reasonable:

Planar plots already used the style sheet so they should be unchanged (and look reasonable).

[xylar]

I retested and all tests still pass.