add themes for JHEP and JCAP

[musoke]

JHEP (Journal of High Energy Physics) and JCAP (Journal of Cosmology and Astroparticle Physics) are physics journals.

Add figure sizes based on the output of

linewidth=\uselengthunit{pt}\printlength{\linewidth}
\\
linewidth=\uselengthunit{in}\printlength{\linewidth}

For JHEP these are

430.20639pt
5.95393in

For JCAP these are

440.0pt
6.08948in

I did not add example figures to the docs - it wasn't clear if these are generated automattically.

[liuyxpp]

Hi musoke, thank you for this PR. However, before I merge this PR, I have some concerns to be addressed:

1. Themes currently in MP are all for publishers (all journals from a publisher share a common theme). I noticed that JHEP is published by Springer. Since journals published by Springer are quite diverse, I don't expect we can generate a common theme for it. But per-journal theme is now not compatible to current MP themes. That means we shall reword related sentences in README and the doc if you want to list JHEP and JCAP. What do you think?
2. Per-journal themes based on a base theme with only some arguments modified have their own source files in your approach. Shall we instead move them into a single file to make the source folder less clutter?
3. I don't see in the JHEP guide mentioning anywhere the figure size requirement (see https://jhep.sissa.it/jhep/help/helpLoader.jsp?pgType=author), the width you provide seems the width of the journal page? Do you think it is appropriate also for its figures？

[musoke]

Hi @liuyxpp, Thank you for the comments

1. Yes, it makes sense to differentiate between publishers and journals.
2. I could put JCAP and JHEP in a single file. What would you like to call it?
3. Yes, neither journal makes an explicit statement about figure sizes. The width I have used is the width of a line of text, not the full page. I have successfully used this in the past (e.g. https://arxiv.org/abs/2301.13220).

[liuyxpp]

I will merge once you fix the following. Many thanks for your contribution:)

1. Can you rewrite related parts of README and the doc in this PR?
2. Maybe journals.jl? Also I can't remember why I name schemes.jl to include all themes. You can rename it to themes.jl in this PR.
3. That's great!

BTW: your next PR is really helpful. Once you fix the corresponding parts which are modified in this PR, I will merge it too.

[musoke]

I have made the changes you suggested. Please let me know if there is anything else.

In principle we could also set the fontsize - this is specified as 11pt in the author manuals for jcappub and jheppub.