Add expected yards after completion (xyac) variables

## Usage

add_xyac(pbp, ...)

## Arguments

pbp

is a Data frame of play-by-play data scraped using fast_scraper().

...

Additional arguments passed to a message function (for internal use).

## Value

The input Data Frame of the parameter 'pbp' with the following columns added:

xyac_epa

Expected value of EPA gained after the catch, starting from where the catch was made. Zero yards after the catch would be listed as zero EPA.

xyac_success

Probability play earns positive EPA (relative to where play started) based on where ball was caught.

xyac_fd

Probability play earns a first down based on where the ball was caught.

xyac_mean_yardage

Average expected yards after the catch based on where the ball was caught.

xyac_median_yardage

Median expected yards after the catch based on where the ball was caught.

## Details

Build columns that capture what we should expect after the catch.