Improve recursion efficiency in check1D and check2D

[mfasiolo]

When check1D is called with a list as argument (e.g. check1D(o, list("x1", "x2", "x3"))) it is then internally called recursively (only one level of recursion) and a plotGam object is returned containing a list 3 ggplots.

However, the residuals are recomputed 3 times (in this case), hence it would be be better to compute them once, and then call the rest of the code. Same for check2D, where this could be particularly important if we are checking all pairs of variables.