Add documentation for chemistry functionalities

[LalitChauhan56]

Fix #677

[LalitChauhan56]

Adding a documentation tutorial shortly.

[isaacsas]

Please make sure to document functions with the normal conventions we use to show function signatures, i.e. see

Catalyst.jl/src/networkapi.jl

Lines 426 to 456 in c74ad14

	"""
	setdefaults!(rn, newdefs)
	Sets the default (initial) values of parameters and species in the
	`ReactionSystem`, `rn`.
	For example,
	```julia
	sir = @reaction_network SIR begin
	β, S + I --> 2I
	ν, I --> R
	end
	setdefaults!(sir, [:S => 999.0, :I => 1.0, :R => 1.0, :β => 1e-4, :ν => .01])
	# or
	@parameter β ν
	@variables t
	@species S(t) I(t) R(t)
	setdefaults!(sir, [S => 999.0, I => 1.0, R => 0.0, β => 1e-4, ν => .01])
	```
	gives initial/default values to each of `S`, `I` and `β`
	Notes:
	- Can not be used to set default values for species, variables or parameters of
	subsystems or constraint systems. Either set defaults for those systems
	directly, or [`flatten`](@ref) to collate them into one system before setting
	defaults.
	- Defaults can be specified in any iterable container of symbols to value pairs
	or symbolics to value pairs.
	"""
	function setdefaults!(rn, newdefs)

Please also add doc strings for the exported accessor functions for querying compounds.

Finally, please add these functions into a new API section on compounds within the docs in addition to the tutorial.

Thanks!

[isaacsas]

Given the API change in #715 we’ll add new docs for that usage.