Add stub unit tests for linear filtering.
Merge branch 'develop' into maint-test-linear-filtering
Use lowercase underscore style and fix here and there.

This is a messy commit.
Fix compilation errors.
Add unit test for `apply_row_based_filter`.
Turn off warning from integer cast.
Define test fixture for linear filtering.
Remove slow implementation of 1D filters.
Use vector instead of raw array.
Change name and signature of column based filter.
Add unit test for row-based filter
Add unit test for column-based filter.
Change signature of `apply_row_derivative`.
Fix the implementation of `apply_row_derivative`.
Add unit-test for `apply_row_derivative`.
Add unit test for `apply_column_derivative`.
Fix implementation of `apply_column_derivative`.
Add unit test for `apply_gaussian_filter`.
Fix the implementation of the Gaussian filter.
Add unit test for Sobel.