Remove redundant functions in SLEPcEigenSolver

[Prof Garth Wells]

No point having:

void SLEPcEigenSolver::get_eigenvalue(double& lr, double& lc) const
{
  get_eigenvalue(lr, lc, 0);
}

void SLEPcEigenSolver::get_eigenvalue(double& lr, double& lc, int i) const;

etc. Remove the first. Clearer if index i is required.

[Jan Blechta]

Agree.

[Prof Garth Wells]

Remove redundant SLEPc interfaces. Fixes #908.

→ <<cset 8ed7dd04e386>>