Transfer grouping - add option to fragment the transfers

[fvdpol]

This PR improves the grouping of USB transfers so the data visualization can optionally support scenarios where the transfer can be extremely long or in case of streaming without an ending; resulting in packets being invisible since these are still stuck in the analysis queue.

A new command-line option has been added to force the Transfers Grouper to fragment the transfer once a specified number of bytes have been successfully transferred. Default is to not fragment, i.e. keep the original behavior.

Example of analysis output for a streaming device with "--fragment-transfers 4096" option:

This PR Fixes #86 -- the issue contains some further details that led to this PR.

[fvdpol]

Marked this pull request as Work in Progress since the collation/grouping of transfers may lead to confusion; will be reworking the functionality to eliminate the confusion/ambiguity.