multibalance.plot issues

[ppanko]

Hello!

I've had some problems with multibalance.plot recently and I would like to propose the following changes:

• When suppling tmatch as the only argument, the function breaks because it's looking for results which does not exist in the scope. I've changed this reference to tmatch, which seems to work.
• Certain instances of matrix subsetting (e.g., covs[,cols]) break if there is only one variable supplied by cols because the object reduces to a vector whereas it is usually expected to be a data.frame. I've changed these to use list subsetting such that the object will always be a data.frame regardless of the number of variables in cols.
• Class checks of the type object == "class" break if the object has multiple class attributes, so I changed them to be more robust (e.g., is.logical/is.factor).

Thanks in advance for your attention!

Pavel