put call to solve() into separate function for testing
adding space for readability
rename variable (solvers will not only be from GSL)
check exit codes from EWSB_solver
Bugfix: set EWSB output parameters to the solver's solution

Before this commit, the EWSB output parameter values from the last
call of tadpole_equations was used.  However, these values don't
necessarily correspond to the best EWSB solution.  The best solution
is the one stored in the solver!
adding test for the four GSL based EWSB solvers