Clarify how CRS is set manually

[AndrewCunliffe]

Re comment on line 101-102 of the code:

Need CoordinateSystem object, but MS only returns 'None' if an arbitrary coordinate system is being used

# thus need to set manually in this case; otherwise use the Chunk coordinate system.

How should this 'set manually'? How about specifying in the readme that the project must be in a meter-based coordinate reference system, and then implementing a check in the script that this is the case?

[h-a-graham]

So All this means (I think) is that rather than defining no crs as None we should set it as: 'LOCAL_CS["Local CS",LOCAL_DATUM["Local Datum",0],UNIT["metre",1]]' This is fine but agree we should add the need for m based system in readme...

[h-a-graham]

This is now solved in a later issue and the comment has been updated accordingly...