rank_k_update: remove ambiguity, #134, #136

[fnrizzi]

@mhoemmen this should fix #136

[mhoemmen]

Thanks for the fix! @crtrott asked us to investigate whether it's possible to constrain alpha so that we can keep the overloads. Would you consider trying out this investigation? I think we would just have to add a requires clause that includes alpha in the algorithm's "mathematical expression" (that's a phrase I define in P1673, meaning "the expression that needs to compile and make semantic sense, for that algorithm").

[fnrizzi]

@mhoemmen yes I definitely I am up to look at it if @crtrott approves me looking at it since he is in charge of my work :)

[fnrizzi]

as soon as I move to blas3, i will take this on