Ampt Upgrade to v2.25 - removal of compiler warnings

[kurtejung]

Upgrade AMPT to v2.25 - additional fixes to remove compiler warnings regarding uninitialized variables.

[cmsbuild]

A new Pull Request was created by @kurtejung (Kurt Jung) for CMSSW_8_0_X.

Ampt Upgrade to v2.25 - removal of compiler warnings

It involves the following packages:

GeneratorInterface/AMPTInterface

@vciulli, @covarell, @thuer, @cmsbuild, @bendavid, @govoni can you please review it and eventually sign? Thanks.
@mkirsano this is something you requested to watch as well.
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@Degano you are the release manager for this.
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