Construct a pivot table over a Spark Dataframe, using a syntax similar to
that from `reshape2::dcast`

.

sdf_pivot(x, formula, fun.aggregate = "count")

## Arguments

- x
- An object coercable to a Spark DataFrame (typically, a
`tbl_spark`

).
- formula
- A two-sided R formula of the form
`x_1 + x_2 + ... ~ y_1`

.
The left-hand side of the formula indicates which variables are used for grouping,
and the right-hand side indicates which variable is used for pivoting. Currently,
only a single pivot column is supported.
- fun.aggregate
- How should the grouped dataset be aggregated? Can be
a length-one character vector, giving the name of a Spark aggregation function
to be called; a named R list mapping column names to an aggregation method,
or an R function that is invoked on the grouped dataset.