Fix Plan::inititalize

There are some issues with recent changes, most notably with:

• 7ddcaf15 Use idg::Array for temp metadata in Plan::initialize
• a84078a4 Plan optimization: channel groups and uvw conversion

The first commit causes weird crashes in the kernels, most likely due to incorrect Metadata. The second commit does not work when nr_channels>1.

This merge fixes the issues while the optimizations are maintained.