Check that min <= avg <= max soil temperature

[dschlaep]

• new tests to check that min <= avg <= max soil temperature at surface and at soil layer depth

• tests for soil temperature at layer depth are currently failing because of min/avg/max soil temperature are not always min/avg/max values in v5.3 #194

[dschlaep]

• R-CMD-check-covr / macOS-latest (release) (pull_request) fails because of issue Github actions are failing if suggested package "rSW2exter" failed to install #198 (which was addressed by commit 224f61d)
• we can merge once this branch points to patched SOILWAT2 v6.5.1 (Bugfix rSOILWAT2 issue194 SOILWAT2#314)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release/v3.5.1@0e6540b). Click here to learn what that means.
The diff coverage is n/a.

@@                Coverage Diff                @@
##             release/v3.5.1     #196   +/-   ##
=================================================
  Coverage                  ?   54.42%           
=================================================
  Files                     ?       42           
  Lines                     ?     6451           
  Branches                  ?        0           
=================================================
  Hits                      ?     3511           
  Misses                    ?     2940           
  Partials                  ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0e6540b...76dab1d. Read the comment docs.