Add two new reaction metadata

[TorkelE]

I have added two more Reaction metadata that we support with getters:

• description
• misc

There are two main uses of this PR:

• These two metadata should be usable by the user to support any metadata they would like to use, without us having to support the addition of user-designated metadata.
• They are good metadata to introduce the concept of reaction metadata in the docs (rather than noise scaling, which is non-obvious what it is). This way I do not have to introduce Reaction metadata in the SDE tutorials.

[isaacsas]

Adding a 'misc' metadata seems redundant? Isn't metadata itself the misc of a Reaction?

[TorkelE]

Yes, ish.

It is redundant if we advice the users that they can add whatever metadata they want themselves. However, since we right now don't do that, I figured this was an easy workaround (without actually looking like it). I.e. whatever metadata a user wants to add they can just stick in the misc field.

In practise, I looked through the metadata permitted for symbolic variables and picked misc and description from that list as they seemed to make sense as easy implementation for Reaction metadata.