make kdir an input to p3_main

[PeterCaldwell]

ECP has the surface rather than the top-of-atmosphere in index position k=1. P3 supports this through the kdir parameter here:

scream/components/scream/src/physics/p3/p3_main_impl.hpp

Line 933 in 90c85ec

constexpr Int kdir = -1;

. We should make kdir an input to p3_main to enable shared use.

ECP probably doesn't need this until June or so.

[bartgol]

Piggy back on this issue: can we change kdir from an int to an enum? Something like

enum ColDirection : int {
  Top2Bot,
  Bot2Top
};

Imho, things that are used for switches and have a limited set of values are better represented by Enum's in C/C++. Using ints seems like a vestige of the original F90 code.

[PeterCaldwell]

Yeah, enum is way better.