Add a basic convolution function

[sjavis]

This aims to create a generic convolution kernel to unify the vertical stencil operations into one. Closes #2539.

• Make BandMatrixRow callable for linear operators.
  • Add basic functions so it works for scalars
  • Use unrolled utilities
• Implement a basic column_convolve! that applies to scalars only

For later PRs:

• Modify the multiplication to use local_geometry either in the broadcasted expression or using a new LinearForm object.
• Allow composition of stencil operations with column_convolve!
• Link column_convolve! into the rest of the code
• Clean up outdated code / operator kernels

Code quality checklist:

• Code follows the style guidelines OR N/A.
• Unit tests are included OR N/A.
• Code is exercised in an integration test OR N/A.
• Documentation has been added/updated OR N/A.