This function adds a distance function to the package. It first
creates the R/{distance_name}Distance.R
file with the R wrapper function
for the distance function. It then creates the
src/{distance_name}Distance.cpp
file with the C++ implementation of the
distance function. It finally opens the latter file in the default editor.
The user will be able to implement the desired distance function in a way
compatible with the RcppParallel workflow.