Find a more meaningful way to to calculate the productivity for productionsites

This closely related to Bug #571796 from 2010.

The percentages that are displayed with production sites when pressing "s" are basically an accounting of successful versus failed attempts to produce a ware for the last 20 such events.
A produced ware increases the percentage by 5%, a failed attempt lowers it by the same amount.
Time doesn't factor in there at all.

In effect, a site that has not produced anything for an hour (economy don't need ...) but had 20 successful production cycles before that time, will still show 100% productivity.
I think that is hard to interpret for newcomers, maybe even misleading and not so useful.

It may be time to overcome and replace this strange calculation with a better, more meaningful solution
that more closely reflects the "real" productivity.

What I consider a "real" productivity, would be a ratio of actual current performance in relation to the site's maximum possible performance, as determined from its conf file. Where performance is measured in "ware output" per period of time.

Advantages:
* It would be easier and more natural to interpret.
* It would more closely reflect the current state of of your economy.
* It would be helpful to decide whether you need to build more similar production sites.
* It would allow to display the percentage without the 5% granularity
* It could also eliminate the disturbing on/off characteristic in the wares statistic display, that you get at higher time resolutions, if that graph was based on these numbers in a separate adjustment.

Example (programers will find a better way to do it)

Each production site should save its successful production events in a buffer together with a timestamp.

The maximum performance for each production site should be calculated:
For this, all relevant time periods should be summed up to calculate the minimum time needed
Example: (Barbarian granitemine)

[work]
consume=ration
sleep=20000
animate=working 20000
mine=granit 2 100 10 2
produce=raw_stone:2

Obviously one production cycle needs 40000 ms and produces 2 raw stones.
That is 20 sec for one stone, or 30 in a period of 10 minutes.

Now look up the site's buffer and calculate how many successful production events it had within a the last 10 minutes.
If there have been 5, the productivity is 5/30 or 16.6 %.
The calculation could be done every minute or so and rounded to full percents.

The maximum performance values will not be exact for those workers that walk a variable distance (lumberjacks, quarry stone masons, farmers) but still more useful than what we have.