Add "Frame" support

[jeisch]

Need to have a way to support multiple "frames", so that multiple events can be stored in a single file.

Should support:

• streaming "frames" out of an existing compressed file.
• streaming "frames" into a new compressed file.
• Making a new, empty frame.
• Dropping a frame instead of writing it.

[brichards64]

So you want to use this a an event storage class. like your own version of a root file. Wasnt the inteded use case, but can be done.

You could however store an event object vector in the Universal storage class. (which is what id prefer as its a universal storage we dont want to over specialise). I can develope this if we want.

As for streaming "frames", currently the file is loaded into memory and the items in the store are interpreted as needed.

If you want to load them one by one from the file we can do that.