generate BD files if fst_vt dictionary BD keys are not empty

[mayankchetan]

@ptrbortolotti: In openfast_io, write BeamDyn files if data structure is available in the fast_vt dictionary (and not only if CompElast=2)

Replaces PR: #2726

We might need extend the ability to write the input files even if the modules are not enabled in the .fst file across to all modules. But, this looks good for v4.0.3.

Tests pass locally:

1/1 Test #156: openfast_io_library ..............   Passed  510.57 sec

100% tests passed, 0 tests failed out of 1

Label Time Summary:
openfast_io    = 510.57 sec*proc (1 test)
python         = 510.57 sec*proc (1 test)

Total Test time (real) = 510.60 sec

[ptrbortolotti]

things are working as intended in https://github.com/WISDEM/TurbOEM
@andrew-platt please include this in 4.0.3