Updates from v0.3.0 release package review

[joshwlambert]

This PR implements a range of minor improvements to the package resulting from suggestions and requests in the {superspreading} full package review (#110).

Updates include:

• current license year
• add internal constants to use in functions and documentation
• improve documentation of ind_control and pop_control arguments
• add .check_input_params() function for repeated check
• add unit tests for .check_input_params()
• add bullet point on constants to design-principles.Rmd