Introduce target values for abundance

[gustavdelius]

The ability to automatically keep biomasses or numbers matched to observations while working in the tuneParams() gadget is very useful. However sometimes one needs to give up on matching the observed values and one wants to introduce alternative target values, while still staying aware of the original observed values. I propose new columns biomass_target and number_target in addition to the biomass_observed and number_observed columns. If they exist matchBiomass() and matchNumber() should match these target values.

Double-clicking on the graphs, which currently only triggers a one-time match to the double-clicked value could in future also set the target value.

Perhaps this target value mechanism can also solve the current problem that arises if both numbers and biomasses are observed and one has to choose to match only one or the other, with having to make the same choice for all species. In future the user could choose whether to create a target value for the biomass or for the number and as long as they only choose one, there could be a matchAbundance() that works for either.