Add feature "operation=sum" to sum data from several periods in the API request

[zawadzinski]

We need to get the sum of statistics data from the “beginning” which means from the moment we added site to the system (ts_created field in site table).

It can be very useful for example in video plugin when we can get the sum of all video events (i.e. video_play) from the first time video appeared online. It is quite important feature.

To do what you want to do, the best thing I can think of is to build a filter that would sum up the DataTable_Array of every year and output only the sum.this could look like:

```
?module=API&method=VisitFrequency.getVisitsReturning&idSite=1&period=year&date=last10&format=xml
&operation=sum
```

This filter could use the DataTable::addDataTable
The only thing to think of is that the DataTable_array could be simple:

```
date=A
data A
date=B
data B
```

looping over the set and sum the datatable should work.

For the version where the rows are themselves made of dataTable_Array

```
idsite=1
date=A
data A
date=B
data B
idsite=2
date=A
data C
date=B
data D
etc.
```

we want the output

```
idsite=1
data A+B
idsite=2
data B+D
```

Keywords: outofscope

[mattab]

Important note: when summing web analytics data, we have to make sure we don’t sum some fields like “nb_uniq_visitors”, as it is not correct to sum uniques

[mattab]

I believe this should be done on the client side. marking as won't fix for now.

[mattab]

Actually this feature is really Custom Date Range since all it does is summing the data and caching it like a normal report, see #572