`fs::Int=` kwarg restriction seems unnecessary

[Dave Kleinschmidt]

the MTConfig struct accepts any type for the fs field (

DSP.jl/src/multitaper.jl

Line 7 in b9c0925

 fs::R1 

), but at least one method that (indirectly) creates one (mt_spectrogram) restricts fs to be an Int: https://github.com/JuliaDSP/DSP.jl/blob/master/src/multitaper.jl#L366