remove String dispatches for MTK constructors

[anandijain]

@ChrisRackauckas and I talked and these would be considered type piracy, even though there are no methods currently taking String in the main MTK repo, other packages may want to define their own

@paulflang I am also thinking that we want to remove the direct ODEProblem constructor, per this comment LCSB-BioCore/SBML.jl#40 (comment)

I think it makes more sense to just require them to use sth like ODEProblem(ODESystem(readSBML(filename))).

I haven't implemented that last point yet though

[ChrisRackauckas]

@ChrisRackauckas and I talked and these would be considered type piracy, even though there are no methods currently taking String in the main MTK repo, other packages may want to define their own

Yeah it's just type piracy. If two packages ever do it, pigs fly and dogs wear hats. It's generally frowned upon and fragile to do that, so instead just define the dispatch on the type SBMLToolkit owns.

[anandijain]

I think I fixed up the tests, but we wont know until the Symbolics stuff is merged in SBML.jl, it seems like this PR makes some of the tests redundant, though I didn't look closely