Update fem1d example, allow variable number of nodes

- Replaced using number of nodes as a global variable with a passable
parameter.
Update fem1d example, remove auto keyword

- Examples should not use the auto keyword.
Update fem1d example, various fixes

- Added comment describing createFem1D function
- Brought stringstream outside of for loop
- Renamed variable holding the quaternion identity.
- Removed scope for creating the scene elements.
- Used three different integration types for the visual examples
- Removed the overlay of the three different integration types.
Update fem1d beam, rename CrossSectionCircular to Radius

- Currently only circular cross section is available for the beam, so we
mention that fact in the class, and we consequently use a less generic
name for the cross section parameter.
Update fem1d beam, move state-based initialization into initialize

- Reduces the number of parameters in the constructor.
Update fem1d beam, remove addForce helper method
Update fem1d beam, reorder methods

- Group the 'add' methods together, and group the 'compute' methods
together.
Fem 1d various fixes

- Reorder headers
- Make header dependencies explicit
- Fix doxygen warnings
- Fix cpplint warnings
- Remove range-based for loop
- Remove empty tests
- Remove superfluous constructor tests