[WIP]: Bandmath#2

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dlindenbaum wants to merge 7 commits intosethfitz:masterfrom

dlindenbaum:bandmath

Commits on Jul 3, 2018

added band1 and band2 to virtual catalog class
dlindenbaum
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added render_nd_png path /ndtiles/ and marblecutter_overload_nd.py for testing
dlindenbaum
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added render overload and bandmath function
dlindenbaum
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render_nd overload
dlindenbaum
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bandmath implementation - no errors but no NDVI image returned
dlindenbaum
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working BandMath
dlindenbaum
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updated README with nd example using 3 bands
dlindenbaum
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