LiPb: fix composition and molar mass calculation

[RemDelaporteMathurin]

Following a report in #305 I looked at the source code of the database:

• the way we calculated the molar mass was wrong
• the composition for eutectic LiPb was wrong for some properties

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.79%. Comparing base (cc4ee9b) to head (b11d842).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #306   +/-   ##
=======================================
  Coverage   98.79%   98.79%           
=======================================
  Files          78       78           
  Lines        1906     1906           
=======================================
  Hits         1883     1883           
  Misses         23       23           

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[RemDelaporteMathurin]

@diaz-alvarez

This is the plot that is now generated by HTM with

import h_transport_materials as htm
import matplotlib.pyplot as plt

lipb_sol = htm.solubilities.filter(material=htm.LIPB)

plt.figure()
plt.gca().yaxis.units = htm.ureg.mol * htm.ureg.m**-3 * htm.ureg.Pa**-0.5
htm.plotting.plot(lipb_sol)

plt.yscale("log")

plt.show()

Is this similar to what you have? Could you share your workflow for easier comparison?