Add docstrings

[mirestrepo]

Many of the functions have some documentation using regular comments (#). Instead those can be changed to DocStrings to improve documentation

[fernandogelin]

@gtollefson Use these instructions (https://docs.julialang.org/en/v1/manual/documentation/index.html) to add docstrings with arguments to all functions. Documenter.jl will take care of adding that info to the documentation.