Performance: Data layers

[polly64]

Layer issues – need to be able to manage multiple nested layers. @matkoniecz is currently adding code for adding transport, admin boundaries, and statistical geography layer options now feels we need to urgently need relook at the way these are structured @polly Hudson, here we could help. You can already assign it to us)
[https://github.com//issues/1334]

[MKIM1008]

@mdsimpson42 contacted me last week to add the layers, too. - I shared the files with Mike.