solve_univariate minor modifications

- make only solveset public
- use warning for solving inequality for complex symbol
- modify docstrings
- use dummy variable rather than importing n from abc and
  add tests to show why it is necessary (to avoid clashing symbols)

  TODO add tests which will test this for the other function in
  which n is used in a lambda.
Merge branch 'smichr-solveset' into solver_refactor

* smichr-solveset:
  solve_univariate minor modifications