Fix calculated values for phasing and prephasing in upload_summary_values.pl

[cbrumme]

This applies to lines 170-196 of upload_summary_values.pl
I've consulted with Illumina and determined that the way that we're calculating the phasing and prephasing values is incorrect.

1. TileMetrics.bin will often contain multiple records per metricCode per tile. We only want to use the LAST value per metricCode per tile. (Note that in everything I've checked so far the LAST value also corresponds to the MAXIMUM, but YMMV)
2. Illumina reports the MEDIAN value for phasing/prephasing, not the Average.