Feat: add gamma distribution for roots

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.48148% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 68.46%. Comparing base (a2d6a2f) to head (5786337). Report is 9 commits behind head on develop.

Files	Patch %	Lines
src/func_discretize_soil_domain.jl	77.27%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #84 +/- ## =========================================== + Coverage 66.26% 68.46% +2.19% =========================================== Files 18 18 Lines 2938 2867 -71 =========================================== + Hits 1947 1963 +16 + Misses 991 904 -87 ```

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