Complex number support for fjåge JSON protocol

[mchitre]

Some languages support complex arrays (e.g. Julia, Python, etc). But JSON doesn't, and Java doesn't. We serialize complex arrays as interleaved real/imaginary parts, which is fine. But we need a way to indicate that the array data is to be treated as complex.

UnetStack uses a mechanism or adding a boolean true field with a suffix __isComplex for complex arrays. For example, if a field signal is a complex array, a field signal__isComplex = true is added to the JSON message.

This works well for most use cases, and is already implemented in Fjage.jl too. I propose that we add this to the JSON specifications for fjåge and implement it consistently across all gateways. Currently the only affected gateway will be Python (fjagepy).

Tasks:

• Update JSON specification (@mchitre)
• Update fjagepy to honor specifications (@notthetup ) - Add support for list of complex numbers for serializing in fjagepy #319

[notthetup]

Also to update Gateway.md to update the spec to mention this.

Done. 8441abe