Remove strain assignment from BGC loaders?

[CunliangGeng]

Strain object is initiated and assigned to BGC object in BGC loaders, like MibigBGCLoader and AntismashBGCLoader.
It makes more sense to init strain objects and assign it to BGC outside of BGC loaders.

• check the logics of strain loading
• check the assignment of strain to BGC, spec and other objects
• assign strain to BGC outside of BGC loaders.

To solve this issue after refactoring of strain mapping loading and GCF loading.

[hechth]

The strain objects in form of a StrainCollection are also passed to the metabolomics loading - I think it makes sense to load the strains afterwards and then add them after loading the metabolomics and genomics seperately.

[CunliangGeng]

Solved in #197.