nystrom factorization for sparse matrix

[changhe3]

fac = nystrom(k, X)

ERROR: MethodError: no method matching nystrom(::PolynomialKernel{Float64,Int64}, ::SparseMatrixCSC{Float64,Int64})
Closest candidates are:
  nystrom(::Kernel{T<:Union{Float32, Float64}}, ::Array{T<:Union{Float32, Float64},2}) where T<:Union{Float32, Float64} at C:\Users\charl\.julia\packages\MLKernels\DqEdF\src\nystrom.jl:97
  nystrom(::Kernel{T<:Union{Float32, Float64}}, ::Array{T<:Union{Float32, Float64},2}, ::Array{U<:Integer,1}) where {T<:Union{Float32, Float64}, U<:Integer} at C:\Users\charl\.julia\packages\MLKernels\DqEdF\src\nystrom.jl:97