Resolve "Add linear interpolator and use it for scaling fields"

What does this MR do?

Add a linear interpolator to select through the InterpolatorFactory.

• Simplify the template signature of Interpolator: Only take DataType and dim as arguments. This also simplifies class specialization for different dimensions.
• Switch to linear as the default interpolation setting for initial conditions in the Richards model.

Is there something that needs to be double checked?

• Is the new template signature fine, or do we actually want the Traits to be included?

Can this MR be accepted?

• Implemented new interpolator
• Added test case for new interpolator to unit test test-interpolators
• Pipeline passing
• Updated user docs
• Update default parameters
• Added entry to CHANGELOG.md

Related issues

Closes #121 (closed)