Extend signal.transform to support Lua functions.

[timrulebosch]

Essentially this:

kind: SignalGroup
spec:
  signals:
    - signal: pong
      transform:
        lua: |
          function signal__CHNAME__PONG(ctx)
              ctx.value = 42
              return ctx
          end  

Using the same technique as for PDU Net.

Also consider initial value or parameter value, would require support in signal vector code.

[timrulebosch]

The transform node will take a form similar to this:

kind: SignalGroup
spec:
  signals:
    - signal: pong
      transform:
        linear:
          factor: 2.5
          offset: 1.0
        functions:
          vector:
            lua: |
              function(s_vector)
                return s_vector * 10
              end
          model:
            lua: |
              function(s_model)
                return s_model + 3
              end