IPython displays a Hamiltonian's str representation instead of repr

[albi3ro]

Currently, IPython displays Hamiltonians via their repr representation:

>>> 1.0 * qml.PauliX(0)
<Hamiltonian: terms=1, wires=[0]>

This doesn't tell us much about what's actually in the Hamiltonian, so I find myself always having to use print(H) instead of simply H when playing around with Hamiltonians in the REPL.

This PR defines the _ipython_display method for Hamiltonians, so now it prints out the full str representation.

>>> 1.0 * qml.PauliX(0)
  (1.0) [X0]

[codecov]

Codecov Report

Merging #2648 (cb85595) into master (e4d86eb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2648   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files         245      245           
  Lines       19724    19726    +2     
=======================================
+ Hits        19642    19644    +2     
  Misses         82       82           

Impacted Files	Coverage Δ
pennylane/ops/qubit/hamiltonian.py	100.00% <100.00%> (ø)

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[Jaybsoni]

Great 👍🏼