Various refactorings

[marcelm]

• Use a std::vector<Syncmer> instead of one vector for hashes and one for positions
• Rename RandstrobeIterator2 to RandstrobeGenerator and add comments to explain what the difference is (to make it clear that we need both at the moment)
• Tiny bugfix: Remove the v prefix from the version in the SAM PG header (only relevant when the version number is derived from a Git tag)
• Rename alignment struct to Alignment
• Move indexing parameters into separate SyncmerParameters and RandstrobeParameters structs

[ksahlin]

Great, makes it much clearer. Approved!