function estimate_flux

[NPJuncal]

estimate the average organic carbon flux to the soil in a indicated time frame (by default last 100 years) from the organic carbon concentration and ages obtained from a age-depth or age-accumulated mass model

it works but there is at least one bug

tasks:

-check resolution of the cores before estimation. If the user wants to estimate the average flux in the last 100 years but the first sample of the core is already more than 100 years older it can not be done. Check that first sample is younger than the time frame provided to estimate the flux. If it is older add warning: " [[core_id]] resolution is to low to estimate the average oc flux in the last [[time frame provided]] years" or something similar and skip to next core

-write test (some already done)

-ckeck documentation

[NPJuncal]

First task is already done. Remainin tasks:

-write test (some already done)

-ckeck documentation

[NPJuncal]

Change flux to sequestration rate in all