Cleaning Heatmap implementation
Cleaning Heatmap implementation

- Factorize get_x_plotting_data and get_y_plotting_data into
getPlottingData

- Factorize get_x_padding and get_y_padding into getPadding

- snake_case to camelCase

- functions to arrow functions

Signed-off-by: martinRenou <a8278cece597ec79cc59974d3d55dbb79bb38416@gmail.com>

ChakriCherukuri/bqplot,SylvainCorlay/bqplot,ChakriCherukuri/bqplot,ChakriCherukuri/bqplot,SylvainCorlay/bqplot,dmadeka/bqplot,bloomberg/bqplot,dmadeka/bqplot,bloomberg/bqplot,SylvainCorlay/bqplot,bloomberg/bqplot