Implement localtopography

[wkearn]

GRIDobj.localtopography computes several different measures of local relief based on min/max filtering (i.e. erosion and dilation), median and percentile filters and mean and standard deviation filters.

Since we will probably prefer to dispatch localtopography in the packages, this issue is more of a meta-issue to track the progress of the tasks required to implement localtopography using libtopotoolbox.

• Maximum filter (Implement dilate #105)
• Minimum filter (Implement erode #108)
• Range filter (uses max filter - min filter)
• Mean (Implement filter #110)
• Median filtering
• Percentile filtering
• Local standard deviation