Non-pressure weighted bunkers storm motion

[aschueth]

As I mentioned in issue #1361, the bunkers storm motion was using a pressure weighted mean wind for its calculations, making the output incorrect. I removed the concatenate(mean_pressure_weighted()) function calls and replaced them with the get_layer function and a simple numpy mean in the line below. I used the output to modify the testing function. I was originally going to use the values from sharppy with a larger range of error, however, after a decent amount of testing, I found out they use the 0-6 shear instead of the (0-500m mean wind) - (5500-6000m mean wind) shear.

• Closes Bunkers motion using pressure-weighted mean instead of normal mean #1361
• Tests modified

[dopplershift]

Thanks for contributing the fix. We'll have to do more in the future when we want this to work on grids of data, but I'm pretty sure it didn't work before anyway.