[WIP] Use block encoding bloqs for all chemistry hamiltonians

[fdmalone]

I might need to split this but given #997 I thought it better to open now to avoid any conflicts:

1. Adds Identity bloq.
2. Makes SF and DF be block_encodings.
3. Replace Reflection (which was actually incorrect) with ReflectionUsingPrepare (needed to add a wrapper bloq to make a PrepareOracle from the Identity matrix (also necessary for block encodings $|G\rangle$ state))
4. [Separate PR] Update walk operator interface.

cc @tanujkhattar WDYT

[tanujkhattar]

Seems like the right thing to do! Also +1 to break into smaller PRs if its not too much work.

Do you want me to leave a detailed review now or are you still working on it? Also, I'm starting to work on a PR that organises utils.py -> bookkeeping/ ; maybe its best to resolve merge conflicts after thats merged? (I'll aim to send the PR in the next 1 hour)

[fdmalone]

You don't need to review now, just saw the organizational PR and didn't want a conflict with reflection. I didn't do anything special when moving the files though.

[fdmalone]

Yeah I can wait to resolve conflicts which should be minor. Should we reorganize reflection first too?

[tanujkhattar]

Should we reorganize reflection first too?

Yes, although I'm not sure where to best put the "reflection" bloq -- my suggestion is to create a new bloqs/reflections/ and then just move the reflection.py and reflection_using_prepare.py to reflections/ -- Although eventually we'd only want reflection_using_prepare.py.

My suggestion is purely based on the aesthetics that we have bloqs/rotations/ and it'd be nice to have bloqs/reflections/ :)

Reflection is used commonly in many applications; so it doesn't feel right it to put it under any specific one like qubitization/, block_encoding/ or amp_amp/ (which doesn't exist yet)

[fdmalone]

I can do this. I don't need octopus right, that's only for splitting up files?

[tanujkhattar]

I can do this. I don't need octopus right, that's only for splitting up files?

Yes, that's right.

[fdmalone]

This was completed in chunks