Adding maxgap to interpolation function

[TommasoGhilardi]

As mentioned in #32, I propose adding a parameter to control the number of interpolated values. I've introduced a new parameter, maxgap, which leverages functionality already available in the zoo package. By default, maxgap is set to Inf, so it doesn't alter the default behavior of pupillometryR. However, users can adjust it as needed to limit interpolation in cases of long gaps.