PlotTopologies plotting improvements (#105)

* Add args to PlotTopologies methods

Add the `central_meridian` and `tessellate_degrees`
arguments to all `PlotTopologies.get_*` methods,
and allow them to be passed as keywords to
`PlotTopologies.plot_*` methods. Default values are
taken from `shapelify_features`.

* Small changes to `PlotTopologies`

Add `tessellate_degrees` argument to calls to `shapelify_feature_lines`.
Also prevent unnecessary reconstruction when setting `time` if the
new value is equal to the current value.

* Automatically determine `central_meridian`

If `central_meridian` is not provided, extract it from `ax`
where possible, rather than assuming 0.

* Improve filling projected polygons

Polygons near the edges of a map projection
should now be filled correctly most of the time.

* Skip line segments with only 1 vertex

* Polygon filling improvements

* Add get/plot methods for topological sections

Allow plotting topological sections (lines) instead of
boundaries (polygons).

* Fix initialising PlotTopologies with `time`

* Polygon edge filling improvements