Change Birkhoff transcription interface to be more consistent with others.

[robfalck]

Proposed feature.

The Birkhoff transcription currently accepts a BirkhoffGrid option specifying its grid data. This is redundant since Birkhoff should only use BirkhoffGrid and never one of the other grid types.

This change will make it so that Birkhoff takes in num_nodes and grid_type and instantiates the grid internally.

Since Birkhoff hasn't been officially released and documented with dymos yet, we will not be preserving the previous behavior.

Example

N./A