TRIM: strip string of trailing blanks (F)

TRIM(string) returns string with trailing blanks removed, e.g.:

 CHARACTER string = " hello world      "
 trimmed_length = LEN( TRIM( string ) ) ! trimmed_length is set to 12

LEN is a function to return the length of a string (or of a REAL array)
```
 full_length = LEN(string) ! full_lenght is set to 18
```
LEN_TRIM is a function to return the trimmed length of a character string
```
 trimmed_length = LEN_TRIM( string )  !  set again to 12
```