Make heat_map module public to expose Smoothing enum.

[aureliegrebe]

Hi,

I noticed there doesn't seem to be a way to access the Smoothing enum in the heat_map module, which is needed as a parameter to the HeatMap.zsmooth() method because the heat_map module is private. I followed what has been done for other modules to expose this enum publicly.

Feel very free to modify if there's a better way to do this. I'm still pretty new to rust.