Insert a new element at a given position in an Rd object.
Rdo_insert_element(rdo, val, pos)an Rd object
an Rd object
the content to insert.
position at which to insert val, typically an integer
but may be anything accepted by the operator "[[".
Georgi N. Boshnakov
val is inserted at position pos, between the elements at
positions pos-1 and pos. If pos is equal to 1,
val is prepended to rdo. If pos is missing or
equal to the length of rdo, val is appended to
rdo.
todo: allow vector pos to insert deeper into the object.
todo: character pos to insert at a position specified by "tag" for example?
todo: more guarded copying of attributes?