Dimensional array read/write support

[YaroslavH2]

I was facing an issue to read/write dimensional array in the variant variable with direct node access.
It looks like there was a TODO placeholder for dimensional arrays in the decoder. But encoding is also missing it.

opcua/ua/binary_decoder.go

Lines 1294 to 1296 in 387afc9

	// TODO: Multidimensional array
	return BadDecodingError
	}

I have made a comit 348ab93 in a forked branch with read/write example working.

It would be useful to have some kind of a structure to keep the information about additional properties of the Variant like this to be part of the variant.:

type VariantMetadata struct {
	//mask contains the type and the array flags
	//bits 0:5: built-in type id 1-25
	//bit 6: array dimensions
	//bit 7: array values
	EncodingMask          byte
	ArrayLength           int32
	ArrayDimensionsLength int32
	ArrayDimensions       []int32
}

I looked into https://github.com/gopcua/opcua library as alternative solution and used some of its array reconstruction method as well as flattened array read/write.

I hope my fork can help to add it here to the master branch eventually.

What do you think?
Let me know how you see it in the end, so I can adapt it for merging...

[awcullen]

That TODO placeholder has been there for years. Thank you for working out a solution.

Why would it be useful to add the metadata fields to Variant?
If we did would users need to use methods like ua.NewVariant() and value.Value() to wrap and unwrap a Variant?

I think your solution is elegant.

[rjboer]

Hi Andrew,

@awcullen, you did a commit, did you fix this?

[awcullen]

I added support to encode a Variant with a 2D or 3D matrix value (from 2D or 3D slices).
I still need to support encoding Structure with a matrix property.
And then I would consider adding more dimensions.