Rework check_input()

[fredjaya]

Related to #27

Usage currently looks like

check_input("pool_size", pool_size)

This is not great as it can be easily misused like

check_input("pool_size", pool_number)

Ideas for improvement:

• pass only the variable and get the "string" within check_input (lame)
• remove check_input entirely, and use the underlying check_* functions. This removes the dependency on each needing to assign a string to the correct function (ideal)

[fredjaya]

Adding and using improved functions to/from R/check_input2.R and replace as needed to avoid changing every single usage in one go.