FlannBasedMatcher::add is overloaded, but the style of parsing the
InputArrayOfArrays does not match the style from
DescriptorMatcher::add. The issue is that InputArrayOfArrays
must be properly marshalled so that the data can be read
correctly. In this case, the method expects the training
descriptors to be either a vector of matrices or a single matrix
(as is shown in DescriptorMatcher::add). These code
replicates that for the case of the FlannBasedMatcher::add.

In fact, a similar commit to this was added by 26d9a7cd but was
ultimately not accepted in #4111. This is likely due to the
fact that the input arrays were not parsed properly and the
case of a single matrix was being improperly handled. I believe
this commit to be correct given the logic from
DescriptorMatcher::add.