Fix stateprep casting rules.

[vincentmr]

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---

Context:
We currently have the following

@qml.qnode(qml.device("lightning.qubit", wires=2))
def circuit():
    qml.StatePrep(np.array([1, 0, 0, 0]), wires=[0, 1])
    return qml.state()

>>> [1 0 0 0]

why causes issues as the integer-valued state is not casted to complex.

Description of the Change:
Introduce ad hoc casting clause for integral types in Lightning.

Benefits:

Possible Drawbacks:

Related GitHub Issues:

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (75701fc) 99.11% compared to head (6a28509) 99.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
+ Coverage   99.11%   99.13%   +0.01%     
==========================================
  Files         141      142       +1     
  Lines       16655    16798     +143     
==========================================
+ Hits        16507    16652     +145     
+ Misses        148      146       -2     

Files Changed	Coverage Δ
pennylane_lightning/core/_version.py	100.00% <100.00%> (ø)
pennylane_lightning/core/lightning_base.py	100.00% <100.00%> (ø)
...ylane_lightning/lightning_qubit/lightning_qubit.py	94.53% <100.00%> (+1.98%)	⬆️

... and 13 files with indirect coverage changes

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[AmintorDusko]

Nice work! Thank you for that!

[mlxd]

LGTM -- thanks