State preparation ops for tensor network

[co9olguy]

Context: The tensor network simulator is currently missing some state preparation ops

Description of the Change: Adding BasisState and QubitStateVector

Benefits: these ops will now be supported

Possible Drawbacks: not sure where in the stack we are checking that state preps happen at the start of the circuit

Related GitHub Issues: #386

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@cdc3776). Click here to learn what that means.
The diff coverage is 76.47%.

@@            Coverage Diff            @@
##             master     #395   +/-   ##
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  Coverage          ?   99.39%           
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  Files             ?       36           
  Lines             ?     2461           
  Branches          ?        0           
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  Hits              ?     2446           
  Misses            ?       15           
  Partials          ?        0

Impacted Files	Coverage Δ
pennylane/plugins/expt_tensornet.py	96.42% <76.47%> (ø)

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[josh146]

Currently the plugins check that state preparation is done as a first operation on a device-by-device basis (and some devices don't even check). It would be great to implement the suggestion in #386