add path admm

[dajmcdon]

• handles numeric overflow in the poisson solver
• adds a path version of the admm in c++
• adjusts the initializer signature (work remains here)
• some adjustments to the main R function admm_solver() which should be renamed.
• converts the internal admm() C++ function to void type (for access to all results) but provides an exported wrapper for testing.
• moves creation of the D matrices to C++. Also allows for irregular spaced data.

This isn't quite complete. The procedure here should be to review (for questions) and also push commits to this branch before merging to main.

Closes #3. Partially closes #4. (should add commits to finish it)