Add ability to use custom norm function in salem plotting

[sunt05]

Add ability to use custom norm function (e.g., LogNorm) in salem plotting

[sunt05]

Thanks @fmaussion for this great package: a must-have when dealing with WRF results in particular for regional maps!
Here is my addition for using a custom norm function as occasionally the underlying levels may need to be manipulated in a more flexible way.
Hope you may consider incorporating this into a future release.

Best,
Ting

[fmaussion]

Thank you very much for the kind words and the contribution. Normally I would ask for a test but this looks simple enough to me, glad that it works for you!

[fmaussion]

https://pypi.org/project/salem/0.3.8/