Convert deprecated '\rm' to '\textrm' in examples

Convert deprecated '\rm' to '\textrm' in examples

This change fixes the export of our FeynmanDiagrams.lyx example
files on the latest TeX Live 2016.

The IJMP(C|D).lyx examples compiled before the change but might as
well convert these instances of '\rm' also.

The changes were made with the help of the following command:

  find ./ -iregex .*lyx -exec sed -i 's/\\rm /\\textrm /g' {} \;

and the resulting diff was confirmed manually.

This commit is consistent with 3cecd4d3.