allow loss metric to be configurable by the user

[jolars]

Possible loss metrics include:

• \sum{i=1}{n} ( (A_i - \omega_i)(A_i )^2 (for A_i != 0).

please comment if there are any other loss metrics that you would want to include

[gwselke]

For me, choice between absolute errors (implemented now) and relative errors (described above) seems enough to cover a wide range of scenarios. One could consider other norms than euclidean, but that would rather seem to be a technical variation without much added practical value. Well, maybe max norm (\max(...) instead of \sum((...)^2).