Add BoundaryHandler and VelocityHandler (#238)

Implements the new `BoundaryHandler` and `VelocityHandler` classes that deal with the issue of particles that don't move in optimisations with boundary conditions. This is achieved by using different strategies that have been described in [Hel10](https://opus4.kobv.de/opus4-fau/frontdoor/index/index/year/2010/docId/1328) (P. 28). These strategies include:
- Nearest bound: Reset particle to the nearest boundary
- Random: Reset the particle to a random position within the bounds
- Shrink: Shrink the velocity such that the particle touches the boundary instead of surpassing it
- Intermediate: Reset the particle to a value between its former location and the bound
- Resample: Calculate new velocities until the next particle position is within the bounds
- Periodic: Tile the space with the same search space over and over
- Reflective: Mirror the position of the particle at the surpassed boundary

Resolves #237

docs/api/_pyswarms.backend.rst

@@ -9,6 +9,7 @@ as GlobalBestPSO and LocalBestPSO were built using the backend module.
 .. toctree::
 
    pyswarms.backend
+   pyswarms.handlers
    pyswarms.topology
    pyswarms.swarms
 

docs/api/pyswarms.backend.rst

@@ -5,20 +5,27 @@ You can import all the native helper methods in this package
 using the command:
 
 .. code-block:: python
-    
-    import pyswarms.backend as P 
+
+    import pyswarms.backend as P
 
 Then call the methods found in each module. Note that these methods interface
 with the Swarm class provided in the :mod:`pyswarms.backend.swarms` module.
 
 pyswarms.backend.generators module
------------------------------------
+----------------------------------
 
 .. automodule:: pyswarms.backend.generators
    :members:
 
+pyswarms.backend.handlers module
+--------------------------------
+
+.. automodule:: pyswarms.backend.handlers
+   :members:
+
 pyswarms.backend.operators module
 ----------------------------------
 
 .. automodule:: pyswarms.backend.operators
-   :members:
+   :members:
+

docs/api/pyswarms.handlers.rst

@@ -0,0 +1,17 @@
+pyswarms.handlers package
+=========================
+
+This package implements different handling strategies for the optimiziation
+using boundary conditions. The strategies help avoiding that particles
+leave the defined search space. There are two :code:`Handler` classes
+that provide these functionalities, the :class:`BoundaryHandler` and the
+:class:`VelocityHandler`.
+
+pyswarms.handlers class
+-----------------------
+
+.. automodule:: pyswarms.backend.handlers
+   :members:
+   :undoc-members:
+   :show-inheritance:
+   :special-members: __init__, __call__

pyswarms/backend/__init__.py

@@ -5,7 +5,8 @@
 """
 
 from .generators import *
+from .handlers import *
 from .operators import *
 from .swarms import *
 
-__all__ = ["generators", "operators", "swarms"]
+__all__ = ["generators", "handlers", "operators", "swarms"]