Write the correct field type to ply

[JoshChristie]

Write the given type to a ply field, instead of always writing Float32.

[JoshChristie]

Ping.

The main difference is that instead of always writing Float32 it will write the type stored in the array.

E.g. The types that I've tested this with are:

Array{N0f8, 1}
Array{UInt16, 1}
Array{UInt8, 1}
Array{UInt32, 1}
Array{Float32, 1}
Array{Float64, 1}

[c42f]

As we discussed a few weeks ago, displaz needs some changes upstream to make this a coherent change. The core problem is that ply doesn't have a way to communicate whether a given field contains fixed point numbers (eg, colors should generally be scaled to be between 0 and 1).