added sumSince (fixes #1843 replaces #376)

[watou]

• added sumSince (fixes persistence extension missing sumSince #1843 replaces add feature sumSince to persistence extensions #376)
• changed averageSince to use BigDecimal instead of double
• changed evolutionRate to use BigDecimal instead of double
• changed deltaSince to use BigDecimal instead of double
• added check in evolutionRate method to avoid divide-by-zero error
• updated all JavaDoc headers to reflect the odd variety of return values and their reasons, in hopes of spurring discussion as to whether and how to tighten and simplify the PersistenceExtensions.

Also-By: Jan N. Klug jan.n.klug@rub.de
Signed-off-by: John Cocula john@cocula.com

[maggu2810]

Overall it looks good.
One remarks:
Can you change the modifiers to the common order (public static instead of static pulic).
The currently used order has not been changed or introduced by you, but as you already touched the function it is perhaps a good time to use a common one. WDYT?
I know there is no "wrong" but there is a suggestion for the order http://checkstyle.sourceforge.net/config_modifier.html

[maggu2810]

Thanks