Add a changelog to track changes between versions

Author: rpep

CHANGELOG.md

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+Version 3.0 Alpha
+------------------
+* Changes to the helper functions init_scalar and init_vector (fidimag/common/helper.py)
+  which allow users to pass additional parameters. These are then used within the sim
+  classes to allow 
+  
+  For example:
+
+  ```
+  
+  mesh = CuboidMesh(nx=10, ny=1, nz=1, unit_length=1e-9)
+  sim = Sim(mesh, Ms)
+
+  def init_domain_wall(pos, domain_centre)
+      x, y, z = pos
+
+      if x < domain_centre:
+          return (0, 0, 1)
+
+      else:
+          return (0, 0, -1)
+   
+  # Place a domain wall at 5nm
+  sim.set_m(init_domain_wall, 5)
+  # Place a domain wall at 3nm
+  sim.set_m(init_domain_wall, 3)
+
+  ```
+
+* Setting currents is now more general, and is standardised across the simulation types.
+  This allows us to use more general functions for setting the current.
+  Previously, the current function was set as:
+  ```
+  sim(mesh, driver='llg_stt')
+  sim.driver.jx = 1e14 # A / m^2
+  sim.driver.update_j_fun = lambda t: np.sin(t)
+  ```
+  with the actual current used being multiplicative:
+
+  $ jx * sin(t) $
+
+  For the current-perpendicular to the plane STT ('llg_stt_cpp') driver
+   we would now change this to 
+
+  ```
+  sim.driver(mesh, driver='llg_stt_cpp')
+  sim.driver.j_function = lambda t: 1e14 * np.sin(t)
+  ```
+  and for the standard STT driver:
+
+  ```
+  sim.drive(mesh, driver='llg_stt')
+  sim.driver.jz_function = lambda t: 1e14 * np.sin(t)
+  # Can also set:
+  # sim.driver.jx_function = ...
+  # sim.driver.jy_function = ...
+
+* Similarly, the TimeZeeman interaction is also no longer multiplicative;
+  you can have an arbitrary function of the form:
+ 
+  def time_function(pos, t):
+      x, y, z = pos
+      # some calculation of Bx(pos, t), By(pos, t), Bz(pos, t)
+      return (Bx, By, Bz)
+  zee = TimeZeeman(np.array([0, 0, 0]), time_function)
+  sim.add(zee)
+
+* You can now remove energy classes from the simulation.
+
+  This can be useful in cases where you have an interaction
+  but no longer need to calculate it because the simulation has 
+  reached a certain point, e.g. an applied field has been turned off.
+ 
+  In the data table, the entries corresponding to a given interaction 
+  will be zero everywhere once the interaction is removed.
+ 
+  
+  For example:
+
+  ```
+  sim.add(Zeeman((0, 0, B), name='Zeeman'))
+  
+  sim.run_until(1e-9)
+  sim.remove('Zeeman')
+  ```