[DOC] Improvement of documentation for Gaps class hierarchy (+GapAnchor).
[DOC] Updates to dox for module bam_io.
[DOC] Updates to dox for accumulators.
[DOC] Improved documentation for module bed_io.
[DOC] Improving dox of CigarElement.
[DOC] Updating API documentation of module random.
[DOC] Improved documentation for seq_io module.
[DOC] Improved documentation of gff_io module.
[DOC] Improved documentation of modifier functors.
[DOC] Doc improvement for graph_types module.
[DOC] Fixing template for inheriting functions etc. from concepts.
[DOC] Improving documentation of stream module.
[DOC] Improving documentation of module alignment_free.
[DOC] Improving documentation of interval tree.
[DOC] Improving bit twiddling function documentation.
[DOC] Improving name store cache documentation.
[DOC] Fixing documentation of SEQAN_IS_32_BIT macro.
[DOC] Improving documentation of sysTime() and cpuTime().
[DOC] Improving documentation of assertion and concept checking macros.
[DOC,FEATURE] More validation and more flexible return types in dox.

Dox validation: classes should not extend concepts.

Also, allowing things like "std::string" as return type.