search configuration parallel

[rrahn]

Description

Adapt seqan3::search_cfg::parlallel to reflect design decisions as discussed in #131.

Acceptance Criteria

• Make configuration element hold directly the member in a variable called thread_count.
• If the option is default constructed and no thread count is set by the user, then during the configuration an exception shall be thrown that the option is not in a valid state.

Tasks

• Adapt the interface to store the variable inside of a variable called thread_count
• Adapt the tutorials/snippets if any
• Add documentation entry on seqan3/search/configuration/all.hpp
• Make the thread_count variable an std::optional and set it to std::nullopt when default constructed.

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