Fix Null and Diagonal preconditioner operator implementation and add test

• Fix null preconditioner implementation for local space
• Fix diagonal preconditioner implementation for local space
• Create a tests/init_nullpreconfields.hpp subclass to reduce code duplication.
• Create a tests/init_diagpreconfields.hpp subclass to reduce code duplication.
• Consistently add test for seg, quad, tri, hex, prism, pyr, and tet.
• Use Nektar++ for reference solution.

Remark: Current implementation of the diagonal preconditioner is very inefficient.

Note: MR!48 should be merged first