Improved support to write named list structures

* src/rmatio.c (set_dims): Now sets ragged = 1 instead of
  returning error for REALSXP, INTSXP, CPLXSXP, and LGLSXP

* src/rmatio.c (write_ragged_struct): Added method to write
  ragged names list structure

* src/rmatio.c (write_struct): Refactoring of function to improve
  support to write ragged list structure

* inst/tests/test_rmatio: Added several tests