Minor fixes and corrections (documentation, vignettes)

• Fixing an issue (wrong calculation for getR0) in the function nosoi::getR0 (linked to issue #14 on GitHub)
• Fixing typos in the documentation (following review of the manuscript)
• Updating website

• Fixing error producing issue following dplyr's update to 1.0.0
• Fixing an issue (wrong attribution of discrete states for internal nodes) in the function nosoi::getTransmissionTree
• Fixing an issue (wrong calculation for discrete models) in the function nosoi::getDynamic
• Reducing dependency on other packages
• Updating website

• Fixing error producing issue following tibble's update to 3.0.0 (also linked to issue #9 on Github)
• Minor fixes and corrections
• Updating website

• Fixing issue #5 (Github) about dependencies; making Suggested packages real suggests.
• Correcting errors due to ggnetwork's use of igraph layouts
• Minor fixes and corrections
• Updating website