TST: Test VAR when constant is already present
ENH: Options for behavior if constant is already present. Closes #2043.
TST: Test new add_constant behavior.
REF: Handle has_constant in model.
REF: Don't change default behavior of add_constant.
BUG: Fix obvious errors for recarrays.
ENH: Add constant checks for ct and ctt.
ENH: Raise on constant for SVAR.
DOC: Add stub for 0.6.1 release fixes.
Merge pull request #2093 from jseabold/add-constant-check

ENH: Add user control over what happens if a constant is already present.