Change wdriftttp to be distribution function shaped.

This ensures we distribute the memory associated with this, potentially large, array. This comes at the cost of using more memory at low core counts. The new approach uses less memory when nproc > nlambda. Cases that don't satisfy this may be expected to be relatively small jobs where resource requirements are less demanding.

This approach may also help avoid a potential bug where the lambda dependent Coriolis drift is added to the totally trapped drift array. Before this commit this would "clobber" any previously set value.

Related to issue #100