Tutorial issues

[smpark7]

Moltres needs more tutorials with better descriptions. This issue can be closed when:

• the main README references the tutorial directory to lead users there
• the READMEs of the existing tutorials are improved for clarity
• the three existing tutorials for group constant generation are consolidated
• a new tutorial directory is created to accommodate this example instead of the current link to a test input file
• new tutorials are created for
  • Eigenvalue calculation
  • Precursor looping
  • Navier-Stokes flow coupling