Fix typos in IDA iterative solvers, add tests

[c-randall]

There were a couple typos in the preconditioner wrappers for IDA:

1. aux_data.r_vec -> aux_data.rvec_tmp to be consistent with IDA_data
2. The prec_solvefn.evaluate() call was missing yp_tmp
3. Documentation referred to spbcg as a linear solver, but code expected spbcgs

I also added test_iterative.py which uses the Robertson problem to test all three iterative solvers. I had to skip these tests for extended-precision builds because I used np.linalg.solve which cannot handle longdouble.