Use 2 timelevels for SCM state variables and remove the leapfrog scheme code

[grantfirl]

SOURCE: @grantfirl CIRA

DESCRIPTION OF CHANGES:

• Use state_X(timelevel=2) to hold the physics-updated state (gt0, gq0, etc) and state_X(timelevel=1) to hold physics-initial state (tgrs, qgrs). Set state_X(1) = state_X(2) at the end of the timestep. Note that this is now more similar to how UFS/FV3 handles the states. It wasn't previously necessary to have 2 different states, but the tendency update physics changes combined with physics schemes that handle prognostic tracers internally necessitate this change.
• Remove the leapfrog scheme since it is no longer needed (hasnt been used since the transition to the DEPHY input format). This is just a cleanup and to avoid future maintenance and confusion.
• Add only to module use statements in SCM code for better readability etc.

ISSUE: Fixes #184

ASSOCIATED PRs:
N/A

TESTS CONDUCTED: SCM RTs

REGRESSION TEST CHANGES: Using 2 timelevels to hold these different states causes machine-precision-level differences.

[lisa-bengtsson]

I just wanted to let you know that I tried this for the prognostic tracers I had included (updraft, area fraction), and it works. Thank you.

[grantfirl]

I just wanted to let you know that I tried this for the prognostic tracers I had included (updraft, area fraction), and it works. Thank you.

@lisa-bengtsson Great! Thanks for following up.

[scrasmussen]

Changes look good! Like the use foo, only: bar changes. Looked through a number of the CI artifact plots that the CI said differed, but couldn't see any change from the baseline.

[hertneky]

Changes look good. I made comments about removal of some leftover mentions of leapfrog in pr718 on accident - mixing up the PRS! I decided to not make the same comments here.
Tested on derecho gnu: built and ran scm rts fine.

[grantfirl]

Changes look good. I made comments about removal of some leftover mentions of leapfrog in pr718 on accident - mixing up the PRS! I decided to not make the same comments here. Tested on derecho gnu: built and ran scm rts fine.

Thanks for the review. I addressed the comments in #718 related to the removal of the leapfrog scheme in this PR. The other comment will be addressed in that PR once we merge this one and update 718.