Add SI_Integrator, SI_CELI_Integrator classes; remove si_celi

The si_celi depletion function has been removed. The
SI-CELI method can be implemented with
    >>> from openmc.deplete import SI_CELI_Integrator
    >>> SI_CELI_Integrator(op, dt, power, stages).integrate()

The stages parameter is optional, and defaults to 10 to
be consistent with the previous default for si_celi.

The SI_CELI_Integrator inherits from the new abstract base class
SI_Integrator. There are a few differences in how the SI-based
methods perform the integration.

- The initial, t=0.0, i=0, no restart transport solution is scaled
  by the number of stages.
- The SI methods also do not perform a new transport solution at
  each successive iteration [t>0, i>0]. Instead, the reaction
  rates are pulled from the last stage of the previous step.
- There is no final transport solution once all the depletion
  steps have been taken. The final reaction rates are saved as
  those from the last stage of the last depletion step.

The si_celi function has been removed from documentation and testing,
and replaced with the SI_CELI_Integrator.

docs/source/pythonapi/deplete.rst

@@ -16,7 +16,6 @@ transport-depletion coupling algorithms <http://hdl.handle.net/1721.1/113721>`_.
    :nosignatures:
    :template: myfunction.rst
 
-   integrator.si_celi
    integrator.si_leqi
 
 .. autosummary::
@@ -30,6 +29,7 @@ transport-depletion coupling algorithms <http://hdl.handle.net/1721.1/113721>`_.
     integrator.CELIIntegrator
     integrator.EPC_RK4_Integrator
     integrator.LEQIIntegrator
+    integrator.SI_CELI_Integrator
 
 Each of these functions expects a "transport operator" to be passed. An operator
 specific to OpenMC is available using the following class: