fastpos 0.3.0

• Parameter for number of studies is now always n_studies. Some lower level
  functions used number_of_studies.
• create_pop now creates a bivariate normal distribution with an exact rho, this
  is slower than previously, but it is probably worth it.
• Problems with C++ progressbar (RcppProgress) were fixed. The code should also
  be cleaner now.
• Make C++ and Rstudio interruption similar: depending on the
  timing, either Rstudio interrupts by itself and stops everything or C++ returns
  a value. In the latter case, R now also stops quietly.
• Since the interrupts cannot be properly tested, code coverage is only 90%.