fastpos 0.4.0

• multiple cores are now supported via the future package, use parameter
  n_cores, gains are only visible if n_studies is high (overhead)
• tests for multiple cores were included
• replace parameter was added to the externally visible functions, and tests
  for replace = false
• minor improvements in layout/documentation
• test added for old pop_create function
• test coverage is now somewhat higher since the untestable proportion of code
  reduced