[NOP] add heavy methyl SILAC labels to unimod.xml

Standard dimethyl chemical labelling modifies Lys and N-termini. In heavy methyl SILAC methabolic labelling [1] Arg can be modified as well. This site is not (yet) allowed for in unimod # 199 and # 510. The pull request adds the following two mods.

Dimethyl:2H(4) (R)
Dimethyl:2H(4)13C(2) (R)

[1] Ong, S.-E., Mittler, G. & Mann, M. Identifying and quantifying in vivo methylation sites by heavy methyl SILAC. Nat Methods 1, 119–126 (2004).