TST: Cleanups in test_theil_sen

- Removed unnecessary arrays e.g. w = np.array([3.])
- Merged two functions, i.e. gen_toy_problem_1d with/without intercept
- Usage of RandomState instead of np.random.seed
COSMIT: Renamed _lse to _lstsq in theil_sen.py
ENH: Removed shared-memory parallelism in theil_sen
COSMIT: Inlined two methods in theil_sen.py

- _get_indices inlined
- _print_verbose inlined
ENH: Use warnings instead of logging in theil_sen
ENH: Removed _split_indices method in TheilSen
ENH: Rewrote TheilSen._get_n_jobs as a function
COSMIT: More explicit names for vars in theil_sen

- n_subsamples instead of n_ss
- n_subpop instead of n_sp