Use `glmgen` in growth rate function when ready

[ryantibs]

When glmgen::trendfilter() is a bit more robust and battle-hardened (is that ever going to happen?), we should use it in the growth_rate() function for the trend filtering option, instead of genlasso::trendfilter().

The glmgen version is going to be much faster, and it would allow for arguments to be more directly passed, just like the smooth spline option (instead of the weird/custom argument handling we have now).