Fix a bug in the Schur eliminator

The schur eliminator treats rows with e blocks and row with
no e blocks separately. The template specialization logic only
applies to the rows with e blocks.

So, in cases where the rows with e-blocks have a fixed size f-block
but the rows without e-blocks have f-blocks of varying sizes,
DetectStructure will return a static f-block size, but we need to be
careful that we do not blindly use that static f-block size everywhere.

This patch fixes a bug where such care was not being taken, where
it was assumed that the static f-block size could be assumed for all
f-block sizes.

A new test is added, which triggers an exception in debug mode. In
release mode this error does not present itself, due to a peculiarity
of the way Eigen works.

Thanks to Werner Trobin for reporting this bug.

Change-Id: I8ae7aabf8eed8c3f9cf74b6c74d632ba44f82581