Bugfix: handle no-element case when printing expression.
Remove various minor warnings.

Warnings are either unused argument (removed parameter name),
unnecessary qualifier (removed const), or recommended braces for
empty else clause (presumably when ASSERT is defined to be empty).
WarningFix: Remove unused local variable m.
WarningFix: removed unused local variable nFR, nFX, nAC, and nIAC.
WarningFix: remove unused variable nFR and nFX.
WarningFix: remove unused variable obj (two instances).
WarningFix: Remove unused variable nAC.
WarningFix: remove unnecessary const qualifier on return type.
WarningFix: remove unused variable cos_2phi.
WarningFix: remove unused variable bound2.
WarningFix: remove unused variable dummy, assign gcvt() return to third arg.
WarningFix: prefixed local shadowing variable names with "ex".
Merge branch 'minor-warnings' of github.com:roryyorke/acado into roryyorke-minor-warnings
Merge branch 'expression-print-bug' of github.com:roryyorke/acado into roryyorke-expression-print-bug