First TurbSim runs fails due to file lock

[phil-brad]

Typically first set of WindGenerationTasks run in parallel fail due to a file lock:

 Using NWTC Subroutine Library (v1.04.01, 21-Feb-2012).

 Running TurbSim  (v1.06.00, 21-Sep-2012).
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 Cannot open file "C:\simulations\IEC\prerequisites\0ca0b3e38711f7fe5c2a0bec150c1d0c\wind.sum".
 Another program like MS Excel may have locked it for writing.

 Aborting TurbSim.

Not currently obvious what's causing this. When those runs are rerun they succeed.