Created new file and added information on gammaray packet initialization

docs/physics/tardisgamma/gammaraypacketinitialization.rst

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+*********************
+Tardis :math:`\gamma`
+*********************
+
+Packet Initialization
+=====================
+
+The gamma ray portion of tardis also uses packets of photons, which are given the following properties:
+- **Packet Energy:** This is the energy of the packet in the comoving frame and is equal to the total energy divided by the number of packets.
+- **Packet Frequency:** This is the frequency of the packet in the comoving frame and is equal to the packet energy divided by Planck's constant, h.
+- **Packet Direction:** This is the direction of the packet and is made up of two angles, :math:`\theta` and :math:`\phi`. 
+:math:`\theta` is a polar angle between 0 and :math:`\pi` and :math:`\phi` is an azimuth angle between 0 and 2:math:`\pi`.
+We randomly sample these angles by using a random numbers z\ :sub:`1`| and z\ :sub:`2`\.
+We then use the equations :math:`\cos{\theta}`= 1-2z\ :sub:`1`\ and :math:`\phi` = 2:math:`\pi`z\ :sub:`2`\ to find the angles.
+- **Packet Times** This is the time that the packet starts propagating the ejecta.
+- **Packet Location** This is the starting point of the packet within the spherical shells of the ejecta.
+This location is given by the equation, v = [zv\ :sub:`inner`\\ :sup:`3`\ + (1-z)v\ :sub:`outer`\\ :sup:`3`\]\ :sup:`1/3`\
+where v\ :sub:`inner`\ and v\ :sub:`outer`\ are the inner and outer velocities of the shell and z is a random number between [0,1).
+Then to get the radial position, r, we multiply this velocity by the packet time. To get the Cartesian coordinates we use the equations:
+x = r:math:`\sin{\theta}\cos{\phi}`  y = r:math:`\sin{\theta}\cos{\phi}`  z = r:math:`\cos{\theta}`