[CMake] Properly exclude header from headercheck.

Add newline at and of file.
[complexmatrixtest] Rename assemble_random_matrix_vectors.hh -> complexdata.hh

As Christoph pointed out in FS1467, that's a rather unusual name for a
DUNE header file, so make it a little more consistent.
[Documentation] Add file comment and formatting hints to complexdata.hh