adding TODOs
removing over cautious assert from mpdata-fct ctor to enable using donorcell with the default options (keeping larger halo)
cleaning up debug leftovers
adding hint_scale option to scale the field just for advection + new unit test (hist_scale); moving ijk into solver_common (was needed to get scaling into solver_common)
correcting a typo!
Merge pull request #142 from slayoo/master

adding hint_scale option + a relevant unit test