Remove `:diffgauge` mode for fixed-point differentiation

[leburgel]

Running fixed-point differentiation using iterscheme = :diffgauge is very slow, while it no longer has any real benefits over iterscheme = :fixed. Therefore, we should probably just remove it (came up in #364).

[leburgel]

If we remove :diffgauge, we should probably also reconsider how the gradient algorithms are configured, in particular the name iterscheme if there's only one, and potentially even rename :fixed to :fixedpoint since that will be the only flavor of fixed-point differentiation that's implemented.

[pbrehmer]

Very much in favor of removing the :diffgauge differentiation mode and also renaming :fixed to :fixedpoint.