Fix decoding of multi-element register payloads

[glopesdev]

HarpMessage.decode sized the payload from the register length while RegisterBase.parse sized it from payload_class. Only array registers declare a length, so for every other kind the first calculation assumed a single element and rejected any longer frame. Device.read and Device.write both end in decode, so this broke every register whose payload spans more than one element, DeviceName on every Harp device included.

decode now reads payload_class.payload_dtype.itemsize, the same quantity parse uses, so there is one derivation and the two cannot disagree. PayloadDecoder declares payload_class in place of length, since every register has a payload class and only an array register has a length.

What a declared length does

Two further defects surfaced in the same area, neither of them filed.

An array register can be declared two ways, by calling a base with an address and a length, or by subclassing with both in the class body. Only the first sized the payload, so the second would inherit a one-element payload and read the first element while discarding the rest. device.olfactometer and device.soundcard both carry registers in that shape. Sizing moved into the metaclass __init__, which runs for either form, and re-declaring a length on a subclass now raises rather than nesting one sub-array inside another.

The schema model defaulted length to 1, so an absent length and a declared length: 1 arrived as the same value and both emitted a scalar. Both model fields now default to None with a minimum of 1, and the emitter branches on absence, so a declared 1 emits a one-element array. A length: 0 is rejected for a register and for a payload member, which the generator reads as absent.

RegisterBase.length keeps its type and its meaning, and no generated output changes. The emitter was checked against generator output for a schema covering an absent length, a declared 1, a declared 3 and struct payloads at both counts, comparing base class, payload type, length, dtype, field names and address, and decoding a frame built by one path through the other.

Where length lives

RegisterBase no longer declares length. It moves to the array register metaclass, so an array register reports its element count and every other kind carries none.

This follows from the decode change above. Once the payload class is the only thing that decides an extent, a length on a scalar or struct register is always None while the payload spans several elements, which invites exactly the mistake this pull request fixes.

Closes #36

[bruno-f-cruz]

One small clarification.