DOC production in soil carbon module: Remove Carbon from TOTSOMC

**Scientific relevance**
Integrate a representation of DOC production in terrestrial ecosystems within the soil column, and export DOC to the MOSART river network via lateral transport.

Related issue #1216

**Scientific background**
The Dissolved Organic Carbon (DOC) production from the soil organic matter carbon (SOMC) pools (depending on the decomposition rate of fast and slow pools) follows a first-order kinetics formulation depending on different carbon pools (see equation 4 in the article [gmd-11-593-2018.pdf](https://gmd.copernicus.org/articles/11/593/2018/gmd-11-593-2018.pdf).

(1)

→ DOC production in soils (KgC/m2/day),
Suffixes → different pools (eg. slow/fast pools),
→ soil layers,
→ DOC production rate (per day),
→ Rate modifier due to soil moisture,
→ Rate modifier due to soil temperature,
→ Empirical factor for a decrease in C decomposition depending on soil layers,
→ Rate modifier due to clay and silt

**Planned implementation**

We target the soil biogeochemistry in CLM.
Eq. (1) produces a DOC flux and will remove carbon from the SOMC (soil organic matter carbon) either from each soil layer separately or from the total column. This implementation considers SOMC as the main source of DOC.

- Declare DOC as a flux type in the file [SoilBiogeochemCarbonFluxType.F90](https://github.com/ESCOMP/CTSM/blob/master/src/soilbiogeochem/SoilBiogeochemCarbonFluxType.F90)

- Add a slim new module [SoilBiogeochemDomMod.F90](https://github.com/devarajun/ctsm-dom-falk/blob/dom/src/soilbiogeochem/SoilBiogeochemDomMod.F90)

At the moment, this module would have only one function: Implement Eq. (1) to compute lateral DOC fluxes from SOMC (same for all patches).

Following variables/parameters are needed

Input: soil temperature, soil organic matter carbon.
Output: DOC lateral flux
Parameters: DOC production rate, Empirical factor for a decrease in C decomposition, rate modifier due to clay and silt

**Possible future extension?**
Extend DOC production for each soil layer separately.
Add soil moisture variability effect on DOC production
Add spatial variation of clay and silt.
Add soil hydraulics.

**Clarification**

Does it make sense to add such a slim new module (similar to e.g. SoilBiogeochemNLeachingMod.F90)?
Or
Should we integrate Eq. (1) into an existing module that is altering SOMC (e.g. SoilBiogeochemDecompCascadeBGCMod.F90)?
Or
Any other thoughts?

People involved @devarajun @ziu1986 @kjetilaas @ecaas