Throw better error when `normalize!` is called for `Histogram` with non-floating point weights

[david-macmahon]

When normalizing a Histogram, the normalize function produces floating point values for the weights. The in-place version (i.e. normalize!) can only be called for Histograms that have floating point weights. When calling normalize! on a Histogram with non-floating point weights the resulting "no method matching..." MethodError is technically correct, but it would be more helpful to display a more informative message. This could be done my defining:

function normalize!(h::Histogram{T,N}, aux_weights::Array{T,N}...; mode::Symbol=:pdf) where {T,N}
    error("Cannot normalize a Histogram with $T weights in-place")
end