The multipole and local expansion share the same implementation. Their
types only differ by a template tag parameter. Consquently, the
`getMultipole`/`getLocal` methods are changed to `get`.

The method change is propageted to the files:

  - FChebKernel,
  - FChebSymKernel,
  - FChebTensorialKernel,
  - FAdaptChebKernel,
  - FUnifKernel,
  - FUnifTensorialKernel,
  - FAdaptUnifKernel,
  - utestChebyshevDirectPeriodic,
  - utestInterpolationMultiRhs,