LayerDescription::addHostFunction API inconsistency

[ptheywood]

Unlike agent functions which can be added to a LayerDescrption using an overloaded method which can take:

• CAgentFunctionDescription
• AgentFunctionDescription
• AgentFn a = AgentFn()
• const std::string &agentName, const std::string &functionName
• const char *agentName, const char *functionName

Host Functions can only be added by a function pointer.

    void addHostFunction(FLAMEGPU_HOST_FUNCTION_POINTER func_p);

Adding an overload which takes a HostFunctionDesciption object would improve API consistency.

(I believe this will have just been missed when HostFunctionDesciption was added during the DAG implementation)