Cache structs to save memory use on repeated calls to _taylorinteg!

[PerezHz]

This PR introduces some cache structs which allow reusing memory on repeated calls to _taylorinteg!.

[coveralls]

coverage: 95.078% (-0.6%) from 95.697%
when pulling d7d4769 on jp/cache
into de3e06a on main.

[PerezHz]

This is ready for review @lbenet @LuEdRaMo

[lbenet]

Thanks @PerezHz for the nice addition. Do you have some comparisons/benchmarks? I'll have a look into it in the next few days!

[PerezHz]

@lbenet I'm trying out some changes in this PR which will help re-use more memory while making this cache concept more user-friendly. I'll run benchmarks once this is ready.

[LuEdRaMo]

@PerezHz: If I create a cache object once and then reuse it, do I have to update de cache.t, cache.x and cache.dx variables before every integration? If so, I think it would be nice to have a function for this.