Revert price to decimal

[nickevansuk]

In line with the discussion here, the below text should be reverted to instead only allow schema:Number as the value of price (not Float, as this erroneously infers floating point representation).

Note that representing price as a Float leads to the possibility of software rounding errors; accordingly use of Text is recommended here.

All implementers and the tooling are currently using numeric representation, and no issues have so far been reported.