Scope raster rc defaults (partial fix for map-utils compatibility)

[cvanelteren]

This PR partially addresses #588 by reducing global Matplotlib rc side-effects that leak into third-party rasterized artists, notably matplotlib-map-utils ScaleBar.

Changes:

• Stop forcing global image.interpolation to none in UltraPlot defaults.
• Stop forcing global savefig.format to pdf in UltraPlot defaults.
• Keep UltraPlot visual behavior by applying interpolation=none locally in UltraPlot image-producing methods (imshow, pcolorfast image branch, spy image branch), preserving user overrides.
• Add regression tests that creating UltraPlot figures does not force global image.interpolation or savefig.format, and that UltraPlot imshow still defaults to none.

[cvanelteren]

See the 2 proposed fixes in #588 -- we can also force the interpolation and them fix the rasterization on their end. We are still leaking the environment which may not be wanted --> not sure how we wouldd clean up ultraplot if matplotlib is imported, i.e. we are permanently changing the RC for them too.