Aggregate over 75 for emigration rates

[kasra-hosseini]

@crangelsmith Two more points about emigration.

1) The emigration curve is non-linear (which can be correct), but it would be good to confirm this (see notebook 5):

2) Emigration has very high values at the first time-steps (see the absolute values in the beginning):

To clarify, the first figure was generated by:

plt.figure(figsize=(15, 10))
plt.plot(time_axis, emig_pop_axis - emig_pop_axis[0], 
         c='k', lw=4
         #marker="o"
        )
plt.xlabel("Time", size=32)
plt.ylabel("Emigrated", size=32)
plt.xticks(size=24, rotation=90)
plt.yticks(size=24)
plt.grid()
plt.show()

and the second figure:

plt.figure(figsize=(15, 10))
plt.plot(time_axis, emig_pop_axis, 
         c='k', lw=4
         #marker="o"
        )
plt.xlabel("Time", size=32)
plt.ylabel("Emigrated", size=32)
plt.xticks(size=24, rotation=90)
plt.yticks(size=24)
plt.grid()
plt.show()