Custom sincospi

Merged Peter Bell requested to merge sincospi into master

I've expanded the sincospi approximations used for my_sincosm1pi0 into the full function implementation based on the stack overflow answer.

Unfortunately, this hasn't fixed the test failures I'm seeing on WSL. Might still be worthwhile if it imporves accuracy anyway? From my runs of there is possibly a marginal improvement but it could just be random noise.

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