Link Tomography should only contain base Bell pair and no purification.

[Naphann]

Motivation

In the early days, QuISP was made to only support link level Bell pair and to test purification protocols. In that sense the handling of purification by Hardware Monitor module makes sense. But now that we want to QuISP be general and more realistic. The link level tomography should be done only on the base Bell pair. Thus the testing of purification protocol should be at the application level (connection level).

This means that all various purification protocols should not live inside Hardware Monitor and if it needs to be somewhere hard coded inside QuISP it should be in Connection Manager.

Suggestion on what needs to be changed

1. num_purification which is used for link-level tomography should be removed and replace num_remote_purification which is currently used for end-to-end connection.
2. initial_purification should be removed.
3. Simplify HardwareMonitor to only concern itself with keeping track of link characteristics.

Answers to possible concerns.

Q: What if the user only wants to test link-level purification? A: We can do this by creating connections from 1 hop away.