Add RATE financial function

[snhv]

Hi,
We already have a bunch of financial functions such as PV, FV, EFFECTIVE, NOMINAL, but not RATE, another essential function in the pack.

Could we have RATE please?
Cheers.

[hanna-kn]

Done (solves pmt(x, ...)=pmt using newtonsolve, with the function expression newtonsolve(pmt("x",\x,\z,\A{0},\B{0})=\y,\C{0.1},"x")).

[snhv]

Thank you so much!