Add Grid/Supply points to map

[slowe]

In DFES 2020 we are adding in "pseudo-Primaries". These will be the non-Primaries contribution within a Local Authority i.e. the values that go to grid supply points. We'll have one pseudo-Primary per Local Authority (where needed). On the map we'll produce a MultiPolygons for each pseudo-Primary by making small rectangles at the coordinates of the supply points within a Local Authority. These will need to be "cut-out" of the existing Primaries map.

Things to do:

• [x] Get list of grid/supply points with locations and which Local Authority they are in
• [x] Create small rectangles around each one
• [x] Add in Blyth Offshore
• [x] Subtract from existing primaries
• [x] Add back into existing primaries with pseudo-Primary names

[slowe]

We may also add in polygons for Blyth Offshore Windfarm using polygons from The Crown Estate Open Data (visualised by the Oil & Gas Authority). These are connected to Blyth 66kV Supply Point so will be part of the Northumberland Direct to Supply Points pseudo-Primary.

[slowe]

@mary-black I have added in the pseudo-Primaries. Here's a close-up view near Blyth so you can see what it looks like. Obviously none of the pseudo-Primaries currently have any data associated with them in the CSVs so they don't get coloured in at the moment. Here's the bar chart showing the "Northumberland Direct to Supply Points" (zero because it doesn't exist in the CSVs).

[slowe]

This is related to issue #49 and I think both can now be closed.