Read all constants from a configuration file.

[primaryodors]

Replace src/classes/constants.h with a module that reads a text file for the values for all the symbols.

The existing symbols would all become global variables, initialized to zero or null. A module would look for a text file with a name like (perhaps) primarydock.conf in the project root folder. This file would also be supplied in the PR. If the file is not found, exit with an error. Otherwise, the values would be read in from that file and the variables would be populated by it.