Add nodes and elements when id is defined using unsigned integer

[ahojukka5]

MethodError: no method matching add_node!(::Mesh, ::UInt64, ::Array{Float64,1})
Closest candidates are:
add_node!(::Mesh, !Matched::Int64, ::Array{Float64,1}) at /home/jukka/repositories/JuliaFEM/src/preprocess.jl:64

I guess the automatic conversion should work, but nid is defined as Int64, maybe Integer would be better?