Miscellaneous improvements

[gdalle]

In conjunction with my review #21, here are some suggestions that should improve your code from a correctness and performance point of view.
There is still more to optimize if you want to, especially regarding allocations. Let me know if you're interested in pursuing this.

Note that I have not performed benchmarks for lack of a meaningful benchmarking instance, but I'm 90% sure that these changes are beneficial. The only downside I can anticipate is a slightly increased compilation latency.

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Correctness

• Replace similar with copy when initializing A and V. This is an incomplete fix for Different results for two consecutive calls to fit! on the same data #22, one would also need to avoid updating epca.V in-place (why do you do it?).

Performance

• Parametrize field types instead of leaving them abstract. This is especially important for function and container fields. See this documentation section to know more.
• Standardize option initializations to Float64 when it makes sense and avoid implicit conversions from Int.
• Explicitly annotate the argument L to force specialization even though it is a function.

Other updates

• Remove outdated dependency on Parameters.jl and bump Julia compatibility to 1.10 (the current long-term support version). As of 1.10, Parameters.@with_kw can be replaced with the built-in @kwdef, and Parameters.@unpack with the built-in (; a, b, c) = object syntax.

[mykelk]

These are great ideas! Thanks so much!

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