Cleaned up isWilson/isStaggered/isDwf/getStencilSteps routines

[weinbe2]

This PR addresses issue #1098 and extends it to the getStencilSteps function as well, replacing various functions that look like this:

bool isWilsonType() const
    {
      return (Type() == typeid(DiracWilson).name() || Type() == typeid(DiracWilsonPC).name()
              || Type() == typeid(DiracClover).name() || Type() == typeid(DiracCloverPC).name()
              || Type() == typeid(DiracCloverHasenbuschTwist).name()
              || Type() == typeid(DiracCloverHasenbuschTwistPC).name() || Type() == typeid(DiracTwistedMass).name()
              || Type() == typeid(DiracTwistedMassPC).name() || Type() == typeid(DiracTwistedClover).name()
              || Type() == typeid(DiracTwistedCloverPC).name()) ?
        true :
        false;
    }

And turning them into Dirac abstract functions so each individual class is "forced" to define it so functions like isWilsonType, etc, don't need to be manually updated each time (or, more realistically, don't accidentally get forgotten).

There are some other functions that could this be applied to, hermitian() and hasSpecialMG(), but it seemed reasonable to leave those as false unless proven otherwise.

This PR now replaces those above functions with one-stop shop static Dirac::[...] functions that can query if various QudaDslashType or QudaDiracType enums correspond to Wilson, dwf, or staggered operators. The various Dirac* classes then query these static functions as appropriate. There is scope to apply these functions elsewhere in QUDA.

This PR also fixes a small bug in split grid for staggered vs asqtad type fermions.

[maddyscientist]

Aside from trivial comment, which can likely be ignored, this is good to go.

[maddyscientist]

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