DMD load methods bugfixes

[ebchin]

This PR addresses two small bugs in the load() methods of NonuniformDMD and DMD.

• DMD::load() loads a trained ROM, but subsequently calling DMD::predict() fails at line 611 because d_trained and d_init_projected are false. This bugfix sets these variables to true when calling DMD::load().
• NonuniformDMD::load() will always attempt to load the optional variable d_derivative_offset. This bugfix ensures the optional file exists before attempting to load.

[chldkdtn]

@ebchin please feel free to "squash and merge"