Need better section names for materials in ProblemDefinition

[jsrehak]

ProblemDefinition defines the following sections for material information:

number of materials
subsection material ID map
  material id file name map
  fuel pin material id file name
  material id file name

These should probably be renamed in a way that makes it more clear what each does.

• material id file name map: map of integers to filenames that contain the material data. (1: mat_1.material, 2: mat_2.material)
• material id file name: filename of the plaintext file containing the layout of materials in the problem, with id values that match the integers in the material id file name map
• fuel pin material id file name: same as previous but for fuel pins.

I recommend we update to something like:

subsection materials
  number of materials
  ids to materials filename map
  filename of material layout
  filename of fuel pin material layout