Adapt to lensfun API backwards incompatible changes

[letmaik]

The next lensfun version (after 0.3.2) has some backwards incompatible changes and other things that should be adapted in lensfunpy: https://sourceforge.net/p/lensfun/code/ci/master/tree/ChangeLog

As lensfunpy exposes some of the breaking functions (Lens.interpolate_*) it will be backwards incompatible as well, meaning the next version will be 2.0.0. For people building lensfunpy from source, this would also raise the minimum supported lensfun version to the next released lensfun version.

The functions with breaking API in lensfunpy are more advanced and probably not used in the majority of cases, which is good.

Given that this is more work than usual and the next lensfun version is still in development and may introduce more breaking changes, I estimate that it will be a good while until the next lensfunpy version.