Add argument for spline boundary conditions

[awkwardPotato812]

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Description
Added an argument bc_type to allow users to specify boundary conditions for interpolating QobjEvo

Related issues or PRs
fix #2098

[hodgestar]

@awkwardPotato812 Thanks for starting this PR. I did a partial review and activated the test run. I will leave a full review for Eric to do though.

[coveralls]

Coverage: 75.43% (+3.3%) from 72.089% when pulling e701ecc on awkwardPotato812:master into 40ff8e1 on qutip:master.

[awkwardPotato812]

I'm changing this draft to the complete PR. I'll update it with any changes related to documentation once the build error is resolved.

[Ericgig]

The approach is good, thank you.
But the variable name should be fixed either boundary_conditions or bc_type. As Simon said, bc_type can be confusing for those not familiar with scipy functions, so let's use boundary_conditions everywhere.

[Ericgig]

Getting close.

[Ericgig]

The new test is failing.

[Ericgig]

Look good, thank you!