Can't instantiate neutral value for non-integer types

[eyalroz]

I mis-designed the reduction and scan functions to take a neutral value as a template parameter; this works for integral types, not for floating-point types (until C++20).

For the time being, this template isn't even that interesting/useful. So - for now we'll just drop it. This will also mean an update of 0.1.0 to 0.1.1.