Add units

[zingale]

We should attach units to quantities using the unyt library

[zingale]

yt-project/unyt#39

[harpolea]

@mojiastonybrook: Looking at https://github.com/python-hydro/pyro2/blob/units/mesh/array_indexer.py in 35f74c4, I've noticed a couple of things:

• The copy function does not copy over the units. Compare the pyro version
  

  pyro2/mesh/array_indexer.py

  Lines 114 to 116 in 35f74c4

  	def copy(self):
  	"""make a copy of the array, defined on the same grid"""
  	return ArrayIndexer(np.asarray(self).copy(), grid=self.g)

  
  vs the unyt version
  https://github.com/yt-project/unyt/blob/679a4473e8944a9a55f7ffc0a72f569e6c23d85e/unyt/array.py#L1683-L1717

• When I tried running the smooth problem using the advection solver, I noticed that the code was failing when it tries to create new ArrayIndexer objects here

  pyro2/advection/advective_fluxes.py

  Lines 98 to 99 in 35f74c4

  	F_xt = u*a_x
  	F_yt = v*a_y

  F_xt and F_yt are defined here as the product of a scalar and an ArrayIndexer object, however they have not been correctly initialized with grids. It looks like unyt does some clever stuff with multiplication in https://github.com/yt-project/unyt/blob/679a4473e8944a9a55f7ffc0a72f569e6c23d85e/unyt/array.py#L1535-L1681 to ensure consistency of units, so we may need to create some wrapper for this that adds the grids back in.
  I managed to fix this by initializing the arrays first:

    F_xt = myg.scratch_array()
    F_yt = myg.scratch_array()
    F_xt[:,:] = u*a_x
    F_yt[:,:] = v*a_y