Audit materials and properties

[craastad]

materials.py contains the various properties of each material (namely rho, c, c_raw, m, and sigma).

Goals of this task:

• add comments to each property in class Material (some added in Fix cost calculations for FliBe first wall #68)
• ensure each property has the correct value
• add units to each property (added in Fix cost calculations for FliBe first wall #68)
• cite a source when defining materials

Reported by @cfe316 stating "The densities of some of the materials seem like they're on the low side".

[craastad]

Some related changes were made in #68 while investigating Issue #65. Please double check and update the c_raw and m for FliBe.