[Alignemnt Configuration] Put align_cfg::debug into align_cfg::detail::debug

[rrahn]

Description

We want the align_cfg::detail subnamespace for alignment configurations.
Here is the proposed change of the interface:

seqan3::align_cfg::detail::debug    [optional]
usage: seqan3::align_cfg::detail::debug

construction - no construction since it is a tag

seqan3::align_cfg::detail::debug;
// this is an instance of seqan3::align_cfg::detail::debug_tag     

access by seqan::get:

auto && var = seqan3::get<seqan3::align_cfg::detail::debug_tag>(config);

check if present:

constexpr bool in_debug_mode = config_t::template exists<seqan3::align_cfg::detail::debug_tag>();

Acceptance Criteria

• Introduce and documented subnamespace align_cfg::detail

Tasks

• Put the debug option into seqan3::align_cfg::detail subnamespace.

Definition of Done

• Implementation and design approved
• Unit tests pass
• Test coverage = 100%
• Microbenchmarks added and/or affected microbenchmarks < 5% performance drop
• API documentation added
• Tutorial/teaching material added
• Test suite compiles in less than 30 seconds (on travis)
• Changelog entry added

[smehringer]

Fixed by seqan/seqan3#2020