Updating C API Entry Points: isFrame -> isDataFrame (#6235)

* change use of isFrame to isDataFrame

* add definition of isDataFrame for older R versions

---------

Co-authored-by: Michael Chirico <chiricom@google.com>

src/data.table.h

@@ -10,6 +10,9 @@
 #if !defined(R_VERSION) || R_VERSION < R_Version(3, 4, 0)
 #  define SET_GROWABLE_BIT(x)  // #3292
 #endif
+#if !defined(R_VERSION) || R_VERSION < R_Version(4, 5, 0)
+# define isDataFrame(x) isFrame(x)
+#endif
 #include <Rinternals.h>
 #define SEXPPTR_RO(x) ((const SEXP *)DATAPTR_RO(x))  // to avoid overhead of looped STRING_ELT and VECTOR_ELT
 #include <stdint.h>    // for uint64_t rather than unsigned long long

src/dogroups.c

@@ -276,7 +276,7 @@ SEXP dogroups(SEXP dt, SEXP dtcols, SEXP groups, SEXP grpcols, SEXP jiscols, SEX
       for (int j=0; j<LENGTH(jval); ++j) {
         thiscol = VECTOR_ELT(jval,j);
         if (isNull(thiscol)) continue;
-        if (!isVector(thiscol) || isFrame(thiscol))
+        if (!isVector(thiscol) || isDataFrame(thiscol))
           error(_("Entry %d for group %d in j=list(...) should be atomic vector or list. If you are trying something like j=list(.SD,newcol=mean(colA)) then use := by group instead (much quicker), or cbind or merge afterwards."), j+1, i+1);
         if (isArray(thiscol)) {
           SEXP dims = PROTECT(getAttrib(thiscol, R_DimSymbol));