Move contrib/chemistry.mac into core STACK Maxima function.

[sangwinc]

Move contrib/chemistry.mac into core STACK Maxima function. Fixes #1764

[sangwinc]

Comments on this would be welcome @aharjula, thanks.

[aharjula]

The thing with this that worries me is that this will trigger a load of a file at execution time. Even though most, if not all other things, are already inside the image, this would not be. It might be better to simply have the whole of "chemistry mac" inside this function.

If I recall correctly, some time ago, half a decade or more, there were cases where people had managed to generate a maxima image, but some logic failed because the image could not load external scripts related to some setting that loaded stuff in this style. That was probably due to issues with the Maxima execution environment, but I think we wrote open those loads to avoid that.

I think it was issue #411 where we removed various loads, in particular those Lisp loads, but I don't recall what the original cause for that was.

[aharjula]

I would do as follows:

1. Write chemistry.mac as a definition of a function (stack_chemistry_declare) that defines all those values and things.
2. And then top-level load that chemistry.mac.

This is to avoid having to go to the file system to load that separately, and instead have that bit already parsed in the image.