ISlice holding on frame pointer causing memory adding up

[HaiwangYu]

Currently seems the frame ident is used here to check if new event comes:
https://github.com/WireCell/wire-cell-toolkit/blob/master/img/src/BlobClustering.cxx#L131

Thinking to save IFrame->ident() in ISlice. @brettviren, if you are OK, I will try this out.

[brettviren]

Hi @HaiwangYu

What I suggest is to make a new type of IFrame implementation that only holds the ident value. The rest of its IFrame API would return empty/null values.

Then, MaskSlice can gain a new configuration parameter (eg drop_traces - but maybe there is a better name). If drop_traces = true then the ISlice that are produced are given this new "ident only" IFrame implementation. Otherwise they get the original input IFrame.

For now, this new IFrame implementation can live right in side MaskSlice.cxx. If we ever need this new type of IFrame in a different node we can expose it by moving it in to the aux sub-package.

[HaiwangYu]

Thanks @brettviren, I will try this out.

[HaiwangYu]

Hi @brettviren, this seems to work. I will make a PR soon.