Providing read access to time member of engine class for actions

[arminms]

In order to implement #6, there should be a way for actions to know the time of the simulation.

[arminms]

One approach is to add a fifth template parameter to action base for time as follows:

template
<
    class NetworkType
,   class ContentsType
,   class ConfigType
,   class RngType
,   class TimeType
>
class action_base

and then initialize it in init() member function:

    void init(
        NetworkType& net
    ,   ContentsType& cnt
    ,   ConfigType& cnf
    ,   RngType& rng
    ,   TimeType& time)

And assign it as a const reference member.

[arminms]

A better solution is to introduce a virtual function to action_base class rather than having a const pointer to time variable:

class action_base
{
public:
    virtual action_base::update_weight(const TimeType& time)
    {    do_update_weight(time);   }

private:
    do_update_weight(const TimeType& time) = 0;
}