Change TagPileup insert size calculation

[owlang]

TagPileup is currently calculating insert size by getting the difference between the inferred start and stop of the whole fragment. This involves using the SamRecord getMateAlignmentStart() function and extending it by getReadLength() if Read 1 maps to the reference "+" strand. This assumes length(Read1)==length(Read2) which may not always be the case so we would like to switch to using the getInferredInsertSize() function to determine the 5' to 5' fragment length.

We will need to reconsider how the midpoint is calculated as well since this assumption is also encoded in how the value is determined. Perhaps also move to base this midpoint calculation on the getInferredInsertSize() function. (relate to considerations weighing use of clipped vs unclipped 5' markers)