Rename `fixed_header` to `fixed_length_header`.
Convert fixed length header items to array.
Clarify handling of first_validity_time et al.
Iteration style tweak.
Fix comment.
Refactor number of integer header items.
Docstrings for data providers.
Clarify (hopefully!) data providers.
Fix error message
Allow override of _COMPONENTS
Flexible data class with relative addressing
Correct mock parameter
Check real-valued header