## What changes were proposed in this pull request?
Changed class name defined in R from "DataFrame" to "SparkDataFrame". A popular package, S4Vector already defines "DataFrame" - this change is to avoid conflict.
Aside from class name and API/roxygen2 references, SparkR APIs like `createDataFrame`, `as.DataFrame` are not changed (S4Vector does not define a "as.DataFrame").
Since in R, one would rarely reference type/class, this change should have minimal/almost-no impact to a SparkR user in terms of back compat.
## How was this patch tested?
SparkR tests, manually loading S4Vector then SparkR package
Author: felixcheung <felixcheung_m@hotmail.com>
Closes#12621 from felixcheung/rdataframe.
a55fbe2a16