[ENHANCEMENT] function naming for conversion

Currently, the conversion back and forth `FusionTensor` and `Array` are done by `to_fusiontensor` and to `to_array`. 

I am not happy with it: these names do not carry the idea of projection between the full space and the singlet space.Ideally, I would like a symmetric name for the back and forth operations. Maybe `project_to_singlet_space` and `project_to_full_space`?
