Implicit Advection paper submission

Test cases for the submission of the paper Adaptively Implicit Advection for Atmospheric Flows by Hilary Weller, Christian K"uhnlein and Piotr K. Smolarkiewicz will be submitted to JAMES

v20.05-alpha

Multi-fluid modelling code release for the 2020 JAMES manuscript submission by
Hilary Weller, William McIntyre and Dan Shipley