separate function to handle missing function arguments

[shahronak47]

• This part of code is repeated at 3 places, hence a separate function has been created for it.

if (!is.null(ppp)) {
    requested_mean <- requested_mean * default_ppp / ppp
  } else {
    ppp <- default_ppp
  }

• Since, two different values are impacted here (requested_mean and ppp) return them as a list from the function.
• Wherever those values are used later in the function used list_name$requested_mean and list_name$ppp respectively.
• Include tests for the new function created.