Minor improvements for signalprocessing.Interp and signalprocessing.InterpCubicSpline

[MothNik]

This pull request provides minor improvements for

• signalprocessing.Interp
  1. fixed potential silent mutation of iava input
  2. fixed typo in docstring
  3. fixed missing cubic_spline in Raises
• signalprocessing.InterpCubicSpline
  1. fixed typo in auxiliary class docstring
  2. fixed bad index in _rmatvec that was silently caught by numpy slicing operations preventing out of bound slicing

Besides, a dedicated dottest test was added for signalprocessing.InterpCubicSpline (adds to the only indirect dottest tests for signalprocessing.Interp(..., kind="cubic_spline").
Also, a test for boundary conditions (bc_type) that are not supported is added to make sure that signalprocessing.InterpCubicSpline raises a NotImplementedError.

[codacy-production]

Up to standards ✅

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[mrava87SW]

@MothNik saw this, will take a look in the next couple of days 😄