Add support for Elm 0.19

[paul-freeman]

Elm 0.19 has been recently released and introduces some significant improvements to the language.

I did some exploratory work and found that most of the code is ready for 0.19 except for the current library used for charting. The author of this package has indicated it will be updated soon. If this is true we can probably update the rest of the PLACE code easily. Otherwise, we may need to replace the charting package - although no other candidates are obvious at present.

[paul-freeman]

The fix to the plotting library has occurred, so putting this on the list of changes for the next release.