TODO

* Migration error messages indicate an index into the migrations list.
  But this may not correspond to the index into the migrations list in
  the input yaml file because symmetric migrations are resolved into pairs
  of asymmetric migrations, and the index in the error messages corresponds
  to the index in the list of resolved asymmetric migrations.
  Maybe demes-python has this problem too?
* Add function to convert to generations.
* Add function to validate existing graph. Useful for checking validity
  after modifying the graph, e.g. converting to generations, or rounding
  times or deme sizes to integer values.
* Check error messages all end with newlines \n.
* Use more specific error codes throughout?
* Line numbers are zero-based, so every error message with a line number
  needs to add 1.
* Return error messages up the call stack to the library's public functions
  (demes_graph_load(), demes_graph_parse()), rather than printing to the
  terminal. This makes error propagation much messier, and we must be
  careful not to allocate memory for ENOMEM errors, but this is the right
  thing to do for a general purpose library.
* Doxygen strings for structs in demes.h
* Check the API docs generated by doxygen.
* Write some documentation.
* Using the non-specific tag specifier, question mark (?), before a value
  results in parse failure without an error message.