There seems to be a gcc 4.7 bug that incorrectly flags the current
3x3 inverse as using uninitialized memory. I'm *pretty* sure it's
a false positive, but it's hard to trigger. The same warning
does not trigger with clang or later compiler versions.
In trying to find a work-around, this implementation turns out to be
faster anyways for static-sized matrices.
```
name old cpu/op new cpu/op delta
BM_Inverse3x3<DynamicMatrix3T<float>> 423ns ± 2% 433ns ± 3% +2.32% (p=0.000 n=98+96)
BM_Inverse3x3<DynamicMatrix3T<double>> 425ns ± 2% 427ns ± 3% +0.48% (p=0.003 n=99+96)
BM_Inverse3x3<StaticMatrix3T<float>> 7.10ns ± 2% 0.80ns ± 1% -88.67% (p=0.000 n=114+112)
BM_Inverse3x3<StaticMatrix3T<double>> 7.45ns ± 2% 1.34ns ± 1% -82.01% (p=0.000 n=105+111)
BM_AliasedInverse3x3<DynamicMatrix3T<float>> 409ns ± 3% 419ns ± 3% +2.40% (p=0.000 n=100+98)
BM_AliasedInverse3x3<DynamicMatrix3T<double>> 414ns ± 3% 413ns ± 2% ~ (p=0.322 n=98+98)
BM_AliasedInverse3x3<StaticMatrix3T<float>> 7.57ns ± 1% 0.80ns ± 1% -89.37% (p=0.000 n=111+114)
BM_AliasedInverse3x3<StaticMatrix3T<double>> 9.09ns ± 1% 2.58ns ±41% -71.60% (p=0.000 n=113+116)
```
- Replace internal::scalar_product_traits<A,B> by Eigen::ScalarBinaryOpTraits<A,B,OP>
- Remove the "functor_is_product_like" helper (was pretty ugly)
- Currently, OP is not used, but it is available to the user for fine grained tuning
- Currently, only the following operators have been generalized: *,/,+,-,=,*=,/=,+=,-=
- TODO: generalize all other binray operators (comparisons,pow,etc.)
- TODO: handle "scalar op array" operators (currently only * is handled)
- TODO: move the handling of the "void" scalar type to ScalarBinaryOpTraits
