Moved calculation of oligo attributes into seperate class.
[x] change filter reference database function to keep and remove options
[x] change fasta loader of reference database to accept a list of fasta files
[x] change fasta loader of oligo database to accept a list of fasta files
[x] move calculation of database attributes (e.g. Tm, GC, etc) from database and filter classes to own class and link to database functions and filters
[x] adapt database and property classes to new structure
[x] update/add doc strings for oligo attributes class
[x] rewrite tests in unittest format for database and property filters
Moved calculation of oligo attributes into seperate class.