estimate_oc_stock: test if stock estimation by carbon mass acumulation had a lower or higher error than predicting carbon density

[NPJuncal]

When the sampled cores do not reach the desired depth, estimate_oc_stock() predictsthe stock from a linear correlation between mass accumulated oc and depth. Many published papers predict carbon density in the missing part of the core and add it to the measured core carbon density.
Test if this method gives lower diferences by using real data from cores longer than the desired depth as examples.