Move r-{desolve,quadprog,pracma,subplex} from maths.scm to cran.scm

[zimoun]

Dear,

The packages r-desolve, r-quadprog, r-subplex and r-pracma are defined
in `maths.scm`. They come from the CRAN archive. For consistency, they
should be defined in the file `cran.scm`. Do you agree to move them?

All the best,
simon