create initial function: `calc_es50()`

[bbest]

Use:

• code
• documentation

[bbest]

Used the following, per Chapter 2 The whole game | R Packages:

# create package
usethis::create_package(".")

# created function in new file R/analysis.R
# added documentation above function as comments with RStudio menu Code, Insert Roxygen skeleton

# make documentation (into man/*.Rd)
devtools::document()

# check documentation
devtools::load_all()
?calc_es50