cumPoolsCreateAGB handles where age==0

[suz-estella]

cumPoolsCreateAGB currently removes all rows of the input AGB table where age == 0 because the Boudewyn equations will not work when age == 0. This update manually sets all "merch", "foliage", and "other" pools to 0 if the total B (biomass) == 0, which allows the function to return pools when age == 0 and B == 0. It will fail if any rows have age==0 and B!=0.

This saves a step in LandRCBM_split3pools where we need to manually re-add the removed rows for every curve with (age = 0, merch = 0, foliage = 0, other = 0).

Tests have been updated to verify that input of (age = 0, B = 0) leads to an output of (age = 0, merch = 0, foliage = 0, other = 0) and that an input of (age = 0, B = 10) will fail.

Other small updates:

• The rows are added to the input AGB table by reference to save memory.
• The 'speciesCode' column is no longer required to be present in the input AGB table as it is not used (though it is OK if it is there)

This requires a small update to LandRCBM_split3pools which I will PR if this change is accepted.