Use optimal prolongation operators

[DWesl]

Description

Provide functions to calculate the optimal prolongation operator of Bocquet et al. (2011?) and Bousserez and Henke (2015).
This will match assumptions made elsewhere in the OI posterior error covariance calculations.

Motivation and Context

How Has This Been Tested?

Added tests of properties mentioned in Bocquet et al. (2013) Bousserez and Henke (2015).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I give permission for my code to be distributed under the existing license

[DWesl]

These methods provide dense matrices. I need to add a function to combine them with the observation operator, test that, then switch the return type so the reduction and prolongation operators are sparse and don't take so long to apply.