Change the colour scale buckets on the DNO view

[dantravers]

For the DNO view, the maximum installed capacity of the regions is 3200MW. Normally a single region might expect in any one half hour to generate of the order of 2/3 of the maximum capacity (I'm judging this by the National forecast, which might get to 11GW, with a GB capacity of 16-17GW).

The current buckets go up to 4500 MW for DNO view (see picture). I'd suggest changing to boundaries:
0
250
500
1000
1500
2000+

This gives the same number of buckets. The smaller first bucket gives some defintion into DNO regions in winter and those with smaller capacities. (We do the same thing for GSPs for the first two buckets).

[dantravers]

As an example - on this day, where National generation was 7,300MW (which is about 2/3 of the max seen), we only get one colour of DNO on the map:

[dantravers]

Last comment! Might look better to use the full numbers in the legend, rather than 2.5k, I.e. 2,500
But something to try out.
Another option would be to use GW, but that might be confusing as we are MW everywhere else.