Improve Cholesky variants

[angsch]

Make the Cholesky variants a bit more competitive by replacing the scalar potf2 with the recursive potrf2. The latter is already used in the primary Cholesky factorization (SRC/dpotrf.f etc.) .

[codecov]

Codecov Report

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[langou]

Thanks @angsch. There is also the question of the variant of the "main" POTRF in LAPACK. Right now I think LAPACK/SRC is using the left-looking variant. This is fine with me, but maybe using the plain recursive variant or the right-looking one would be better. (Preference for recursive.) I am comfortable if we leave things as they are.