mesh_plotter improvements

[samsrabin]

Description of changes

• Adds --dpi option (default 300)
• Existing output file without --overwrite will now actually fail
• More reasonable cell-center marker sizes, with an option to disable them

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed (include github issue #): None

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? Yes:

• Adds --dpi option (default 300)
• Adds --no-center-coords option; useful for when the red X's are uselessly small or way too big

Does this create a need to change or add documentation? Did you do so? No

Testing performed, if any:

• Options tested manually
• New Python system/unit tests added
• Python unit testing passes
• Python system testing passes

[adrifoster]

This looks great to me. Some nice pythonic additions and improvements to code robustness.