Remove preprocessor switch float vs double

[MatthewPeterKelly]

The key change in this commit is to replace the preprocessor flag that switched between using float and double precision.

It does not make any change to the implementation or documentation.

There were several small code clean-ups included as well:

• use standard Eigen typedefs (rather than custom)
• remove "using namespace Eigen"
• fix the scoping on local constants
• replace #define INF with numeric_limits<>::max()