Dev v2f improvements

[shaering]

Series of improvements to v2f and other small changes:

1. improved smoothed transition functions,
2. reinstated f and Pk filtering,
3. removed inconsistent streamwise-stabilization from Qt calculations (Qt is the normalized material derivative of temperature and computed from the rhs of it's evolution eq, i.e. there is no convection and no streamwise-stabilization),
4. added streamwise stab to v2f,
5. added joule heating guards to prevent heating outside of the torch, and
6. sparking capability to reacting flow