Move all plotting functions into the utils plotting modules

[john-science]

ARMI has several plotting tools scattered throughout the codebase:

• armi/bookkeeping/report/newReports.py : def plot
• armi/nuclearDataIO/xsNuclides.py : def plotScatterMatrix
• armi/nuclearDataIO/xsNuclides.py : def plotCompareScatterMatrix
• armi/reactor/blocks.py : def plotFlux
• armi/reactor/converters/blockConverters.py : def plotConvertedBlock
• armi/reactor/converters/geometryConverters.py : def plotConvertedReactor
• armi/reactor/converters/uniformMesh.py : def plotConvertedReactor
• armi/utils/__init__.py : def plotMatrix

These should be moved into one of our plotting modules

• armi/utils/plotting.py
• armi/utils/reportPlotting.py

If it helps speed things up, we can preserve the current API by moving the the potting functions to utils/, and then just importing them in the modules where they are now.