Formalise the description of operator argument types

- Standardise the terminology for operator arguments. Argument,
  Operand, and Parameter were used interchangeably.
- Introduce a templatized tensor_t<> type for tensor arguments.
  Scalars are represented by rank-0 tensors.
- Types can be checked with the XSD schema.

Signed-off-by: Kevin Petit <kevin.petit@arm.com>
Change-Id: Ic57b9387950824e994c5e7f9ec1489c29159b974