Inconsistent tocoo/to_coo methods on different sparse types

[mangecoeur]

There is some inconsistencies in the naming of to-coo methods: GCXS has tocoo, DOK has to_coo. I assume to some extent it's to have some compatibility with scipy sparse. But then why not just have both methods on all types?

[hameerabbasi]

We're considering deprecating those and using asformat always instead.