Fix division by zero in GLCM

• GLCM: properties can ignore nan values to compute properties dimension reduction with stats.
• GLCM: division by zero returns zero instead of inf.

GLCM module

• GLCM: new module stats.glcm.
• Pelt: bug fixes.
• Filter: bug fixes and new filter type.
• Neighborhood statistics: user Kernel to compute stats, move to numba for loops.

Filter and Stats neighborhood submodules

• Filter: Filter dataset by values.
• Neighborhood statistics: Compute statistics over dataset variables.
• Pelt: Output as dataset. Dates as unix time.

Fix Pelt

• Pelt: Fix execution with xarray.
• Docs: More detailed installation instructions.

Pelt submodule

• Kernel: Add a class for Kernel objects and utility functions to create kernels of different shapes.
• Pelt: Add function to compute the Pelt algorithm over xarray DataArrays.