improve test coverage for impl.Mesh and products.SufracePrecipitation; introduce Mesh.domain_bottom_surface_area; switch from volume to mass for rainfall counting; improve docstrings (default length of omitted dims, liquid/ice phase). closes #708

improve test coverage for impl.Mesh and products.SufracePrecipitation; introduce Mesh.domain_bottom_surface_area; switch from volume to mass for rainfall counting; improve docstrings (default length of omitted dims, liquid/ice phase). closes #708 #1870