BUG: Fix rounding issue with HistogramPlotter test

This patch addresses the following failing test:
http://open.cdash.org/testDetails.php?test=222652964&build=3137224

Change-Id: Id023b2dfa32e0698b06ab4413e78f8826d19fd38
COMP: Remove unused method to manage warning

This patch addressed the following warning

ImageRegistrationHistogramPlotter.cxx(242): warning #177: function
"<unnamed>::HistogramWriter::GetMetric" was declared but never
referenced
    MetricPointer GetMetric() const

Change-Id: Id2e5c4f309751d3ad65adf852f512c3d0b00dd0b
Merge topic 'FixHistogramPlotterTest'

15c7f90 BUG: Fix rounding issue with HistogramPlotter test
Merge topic 'FixUnusedGetMetricPlotter'

4be315d COMP: Remove unused method to manage warning