Increase const-correctness in src/coordinate.cpp

[Richèl Bilderbeek]

Sometimes things go wrong. For example, in Pull Request #731, Claudio got reviewed by Richel. Richel suggested to add const in some places, yet accidentally accepted the review and even merged the code. In this Issue, his mistake is undone :-)

Look at #731 where Richel suggested to use const int height{2000}; and const int width{1000};. Do those suggestions in the code.