Source folder restructure

[TorkelE]

Moves around functions in the src folders:

• Renamed "reaction_network.jl" to "dsl.jl".
• Split the "reactionsystem.jl" and "networkapi.jl" files into 4 files (one for the Reaction struct, one for the ReactionSystem struct, one for conversion, and one for network analysis).
• Moved some constant declarations from the "dsl.jl" file to the "Catalyst.jl" file.
• Moved some functions from the "dsl.jl" file to the "expression_utils.jl" file.
• In many files, reorder function order to group them more naturally.
• Added some comments and section comments (generally very minor).

[isaacsas]

Seems like there are still some places that functions got moved to that are after where they are called (sometimes into a different of the new sections).