amountByGroup - needs further developments/improvements

[anacfernandes]

• the function is not working with subsets of the data -> need to include filters in the parameters of the function, if we want to visualise the information e.g. at stock (species specific) level or by quarter.
• at this moment, the function is considering frequencies of values for SA and BV tables and not sums (e.g. total numbers or total weights) - need to change this (for SA table, it doesn't seem to make sense; for BV we need to investigate more, because it might depend on the biological variable of interest).
• create the code to produce the outputs in the 'table' format;

[rix133]

needs a vignette and tests