Null FF in createing relative vorticity

[ghansham]

Dear Sir

The following line has uFF and vFF as null when UisSequence is false in createRelativeVorticity method.

IDV/src/ucar/unidata/data/grid/DerivedGridFactory.java

Line 526 in 112f1a3

rvFI = (FieldImpl) GridMath.ddx(vFF).subtract(GridMath.ddy(uFF));

Ghansham