Add mc2 aggregation methods for biological replicates

[madison-feshuk]

Currently all rvals are always treated as technical replicates with no aggregation to biological replicates. Aggregation to biological replicates must occur in pre-processing steps. To address this, proposed new mc2 methods include:

• bio.rep.mean: calculate cval as mean rval across tech replicates
• bio.rep.median: calculate cval as median rval across tech replicates
• bio.rep.percent: for count data, calculate cval as sum of rval / tech replicates * 100

Need to add logic for requiring minimum replicates (>1) at a concentration for zebrafish data (Could be second method)