NTR: ploughing process

[pbuttigieg]

Linked to #33
We'd like a class to express ploughing activities to axiomatise arable land.

This would need the creation of a class for plough (syn: plow) too

proposed def: "A process during which a plough is mechanically drawn through soil to prepare it for seed sowing or planting"
dbxref: https://en.wikipedia.org/wiki/Plough
syn:

[celineaubert]

ploughing process = tillage. I have added the synonym to the class ''Tillage''