implement get_defaults(mdl::SBML.Model) method

[paulflang]

I am trying to modify the kwargs in ReactionSystem and ODESystem, but it does not work. It actually feels a bit fishy to do this. Any suggestions on style/how to get this work?

[paulflang]

also, we need the commits in #16 for this to work. @anandijain can you please comment there or here why you reverted #16 ?

[anandijain]

there is no need for a get_defaults(::Model) particularly in this repo. we want to keep this as much of a shim package as possible

[paulflang]

I disagree. Our package is to import SBML. It is very basic functionality to extract the ReactionSystem, u0map and paramap and thus dafaults separately from the file. In my opinion there is no better place to put these functions than SBMLToolkit. get_u0map, get_paramap and get_defaults are MTK specific, so they definitely don't belong in SBML.jl. They also don't belong in MTK, cause they extract from SBML.Model.