Allowing redundant 'as-is' operator for MTR specifications

[johnnybonney]

I've noticed that the package does not allow me to specify an MTR formula that contains the term I(u). ivmte throws a (very informative) error:

Error : The following terms are not declared properly.
  m0: I(u) 
  m1: I(u) 
The unobserved variable 'u' must be declared as a monomial, e.g. u, I(u^3). The monomial can be interacted with other variables, e.g. x:u, x:I(u^3). Expressions where the unobservable term is not a monomial are either not permissible or will not be parsed correctly, e.g. exp(u), I((x * u)^2). Try to rewrite the expression so that 'u' is only included in monomials.

Using the as-is operator this way is clearly redundant (that is, I could just use u instead of I(u)), but other standard R functions (lm, glm ivreg, etc.) allow for such redundant usage.

This is simply a user-friendliness thing and may not be something you want to update. If you don't think it is worth the effort, feel free to disregard!

[jkcshea]

Ah sorry, yes, I can do this!