snippet(latex): Add (texmathp) condition to certain snippets

[itxprashant]

Fix: #c63094b

• Added (texmathp) condition for math specific snippets.
• Added yasnippet-mode header for int, sum and prod.
• Removed file-ending newlines.

[itxprashant]

Kindly merge this commit and delete the previous one, because it introduced some new-lines bug.

[hlissner]

Thanks again!

[itxprashant]

Don't want to bother you, but the newlines omission hasn't been merged yet, that's why I intended for deletion of previous commit and rebasing this one.

[hlissner]

Ah, my bad! I don't generally like to force-push on commits that are more than 30 minutes old, so rather than delete the old commit, I've overwritten ed571b1 with cacd22f, which includes the newlines fix (and a more correct commit message). Thanks for letting me know, in any case!

[itxprashant]

Thank you so much!