Take care of parameter estimates at prior bounds in check plots

Symmetric plots for parameters at the prior bounds resulted in nothing being printed. This commit then switches to asymmetric plots for those parameters and plots a red line signifying the bound.
Bugfix for correct display of names of parameters outside of bounds
Display source of non-positive definite Hessian if parameters at prior bound
Merge pull request #441 from JohannesPfeifer/mode_check

Bugfix for mode_check and display of parameters out of bounds