Speed up test_driver CircleCI plan (#335)

## Purpose

This PR updates the test_driver CircleCI plan to cache its gt4py caches for faster execution, and reduces the simulation time of the caching comm example (which is run as part of CI).

## Infrastructure changes:

- test_driver circleci plan now caches .gt_cache
- read/write examples for caching comm now only run two timesteps instead of one hour

.circleci/config.yml

@@ -355,12 +355,22 @@ jobs:
           name: Install Submodules
           command: git submodule update --init
       - setup_environment_mpi
+      - run:
+          name: save gt4py_version.txt
+          command: git submodule status external/gt4py | awk '{print $1;}' > gt4py_version.txt
+      - restore_cache:
+          keys:
+            - v1-gt_cache_driver-{{ checksum "gt4py_version.txt" }}
       - run:
           name: run tests
           command: |
             . venv/bin/activate
             cd driver
-            MPIRUN_CALL="mpirun -n 6 --mca btl_vader_single_copy_mechanism none" make test_mpi
+            GT_CACHE_ROOT=$(pwd)/.gt_cache MPIRUN_CALL="mpirun -n 6 --mca btl_vader_single_copy_mechanism none" make test_mpi
+      - save_cache:
+          key: v1-gt_cache_driver-{{ checksum "gt4py_version.txt" }}
+          paths:
+            - .gt_cache
 
   test_driver_orch_cpu:
     docker:

driver/examples/configs/baroclinic_c12_comm_read.yaml

@@ -17,8 +17,8 @@ comm_config:
     rank: 0
 nx_tile: 12
 nz: 79
-dt_atmos: 225
-minutes: 60
+dt_atmos: 180
+minutes: 6
 layout:
   - 1
   - 1

driver/examples/configs/baroclinic_c12_comm_write.yaml

@@ -18,8 +18,8 @@ comm_config:
       - 0
 nx_tile: 12
 nz: 79
-dt_atmos: 225
-minutes: 60
+dt_atmos: 180
+minutes: 6
 layout:
   - 1
   - 1