Load all .R files in ~/.orgR folder

* lisp/ob-R.el (org-babel-expand-body:R): added functionality to
  create attached 'org:functions' environment load
  all files in ~/.orgR/*.R. In these .R files, the functions used for
  variable transfer have to be defined:
  - .org.createEnvironment  (                               env)
  - .org.assignElispTable_1 (name, file, header, row.names, env)
  - .org.assignElispTable_2 (name, file, header, row.names, env)
  - .org.assignElispValue   (name, value,                   env)
Cleanup of redundant code

* etc/R/org_functions.R: deletion of duplicate code and removal of
  code to create 'org:functions' environment upon load