Refactor "src/dsl.jl" file

[TorkelE]

Also does the "src/expression_utils.jl" file, as that primarily support the dsl file (the other relevant file is the chemistry one).

Primarily four types of changes:

• Added comment to code which did not have (primarily older code).
• Rewrite some old code (a lot of stuff in the dsl file is stuff I wrote over 6 years ago, which simply can be rewritten better now).
• Do some organisations for where the options are handled in the main function. Been meaning to do this pretty much since I introduced them, and now when we have all *for the foreseeable future) it seemed like a good time to do it. Now it should also be easier to see where to add new options in the future.
• Rewrite so that the @reaction_network and @network_component macros reuse minimal amount of code (where as previously it the second was basically ctrl+copied versions of the first.
• Add a bunch of error checks to handle things which previously went through but shouldn't.

I also add some tests of all the various ways to create models via the DSL, mostly stuff that was not tested before.

[TorkelE]

Part of the organisations so that options are handled together (in two sets, some before and some after the reactions are considered, depending on whichever is possible for which option)

[TorkelE]

in some places we used ex and some expr, here I just made it uniform

[TorkelE]

This should be ready now. However, Plots fails on precompilation, so have to wait until that is fixed.

Not high priority though, doesn't actually change anything (but should make further updates to the DSL easier).

Once this PR is merged, I will have a follow-up one which reorders the option handling functions to follow the order with which they are called.

[isaacsas]

Please check that non-scalarization is still working ok for vector parameters, and ensure there isn't a performance regression. See the examples in #1051

[isaacsas]

Was this added by someone though who wanted a test to check they could get the expressions out and evaluate them? If so we should probably keep some kind of variation on this checking such a process works.

[isaacsas]

Could we modify the test to eval within a new module? Maybe that would avoid the observed issue?

[isaacsas]

Ahh, nevermind that comment. For some reason Github showed me an old version of the code... Looks good to me.

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