Generalize preprocessing on fx variables used for preprocessor masks

[valeriupredoi]

Natural extension of #439 to the fx variables (files) used to build landsea, landseaice and weighted masks. The only catch here is to allow non failures if fx files are not found since NE masks are applied.

[valeriupredoi]

I will tackle this heroically after #439 has been merged - so giddyup and get that one in 🍺

[bouweandela]

Depends on #436. There will be much better support for ancillary variables in iris 3. Removed this from the v2.0.0 milestone, since the release of iris 3 is scheduled after the ESMValCore v2.0.0 release.

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[schlunma]

This is implemented now (#1609).