Add a :LAYOUT read/write table

[coretl]

After DiamondLightSource/pymalcolm#390 then *METADATA.LAYOUT will contain JSON like {"COUNTER1": {"x": 1.2, "y": 2.3}, "TTLIN1": {"x": 0.0, "y": 5.6}

This should be read into an NTTable with PV :LAYOUT, looking like:

Block	x	y
COUNTER1	1.2	2.3
TTLIN1	0.0	5.6

This should accept puts that will JSON encode the contents and write it down to *METADATA.LAYOUT