Remove `elemental` from `add`

1. Fortran 2003 standard (http://www.j3-fortran.org/doc/year/04/04-007.txt) says:

    > C1277 All dummy arguments of an elemental procedure shall be scalar dummy data objects and shall not have the POINTER or ALLOCATABLE attribute.

2. `elemental` causes broken output for deferred length character arguments when compiled by IFort (GFortran is OK)
3. Element-wise concatenation of character arrays might be a very rare use case