Fix error estimate

Fixes #89 (closed)

Changes made in this Pull Request:

• Use correct error propagation in EpsilonPlanar

Error looks reasoanble now (blue bars are the errors).

Code for the plots

datapath = "../../../../tests/data/airwater"
u = mda.Universe(f'{datapath}/topol.tpr', f'{datapath}/traj.trr')

eplan = maicos.EpsilonPlanar(u.atoms).run()
                 
plt.errorbar(eplan.results.z,
             eplan.results.eps_par.flatten(),
             yerr=eplan.results.deps_par.flatten())
plt.plot(eplan.results.z,
         eplan.results.eps_par.flatten(),c="red",zorder=10)

plt.ylabel("eps_par")
plt.xlabel("z")
plt.show()


plt.errorbar(eplan.results.z,
             eplan.results.eps_perp.flatten(),
             yerr=eplan.results.deps_perp.flatten())
plt.plot(eplan.results.z,
         eplan.results.eps_perp.flatten(),c="red",zorder=10)

plt.ylabel("eps_perp")
plt.xlabel("z")
plt.show()