Add methods for retreiving number of iterations and residual norm from KSM linear solvers

[A-CGray]

In my nonlinear solver, I wanted to be able to check the final residual and the number of iterations from each linear solve (e.g to decide whether the preconditioner needs to be updated), this PR adds this functionality for all solvers that inherit from TACSKsm

PR Summary:

• Adds protected iterCount and resNorm attributes to the abstract TACSKsm class, iterCount is the total number of iterations taken for the last linear solve and resNorm is the residual norm from the end of the last linear solve.
• Adds default constructor to TACSKsm that initialises iterCount and resNorm to 0
• Adds getter methods for iterCount and resNorm to TACSKsm and their python interfaces
• Adds code to update iterCount and resNorm to the end of the solve methods of the PCG, GMRES and GCROT classes that inherit from TACSKsm
• Adds basic convergence tests that rely on iterCount and resNorm to StaticTestCase.test_solve

Closes #4

[gjkennedy]

A few minor corrections

[A-CGray]

@gjkennedy are you happy with merging this now?