Rename "period" in statistics output to "start year"

[jamaa]

The "period" output e.g. in annual statistics can easyil be interpreted as meaning the hydrological year. But e.g. in Germany the hydrological year is defined as the year following the 1st of november in which it starts. This can be defined differently in other places though.

To avoid confusion, "period" should be renamed to "start year"