Percentile calculations need to use a better default multiplier

[Shawn Laffan]

The percentile calculation uses a default precision multiplier of 100. This works for most data, but not when the values are very low (<0.001).

One option is to use the mean of the raster as a starting point. Another is to let the user define the precision (currently this is not exposed in the tools).

See discussion in issue #16 for more details.

[Shawn Laffan]

Updated in commit 3191a97eb3dda4df74195d9b36039a96ed3ecb27

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[Shawn Laffan]

Updated in a2dfd70 and 3f6e6e7

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[Shawn Laffan]

Issue is moot now we use numpy for the percentiles.