Find alternative to na.omit

[mfasiolo]

Currently qgam calls na.omit( data ), but this is a serious problem because data might contain a column full of NAs, which is not included in the fit. Hence excluding the corresponding rows does not make sense.

[mfasiolo]

This commit 3bee94a should have fixed the problem, we now select only the relevant columns of data, and then we drop out the rows containing NAs.