Issue 812 - Implement point_partial_derivatives for dim3

[icui]

Description

• Re-organize operator location
• Add 3D specification and instantiation

Issue Number

Closes #812

Checklist

Please make sure to check developer documentation on specfem docs.

• I ran the code through pre-commit to check style
• My code passes all the integration tests
• I have added sufficient unittests to test my changes
• I have added/updated documentation for the changes I am proposing
• I have updated CMakeLists to ensure my code builds
• My code builds across all platforms

[icui]

retest this please.

[lsawade]

LGTM

[Rohit-Kakodkar]

Are you implementing the accessor and cointainer concepts, discussed within #832 , within this PR ? The enum classes for accessors could be part of this PR.