`linrange` for constructing ranges by endpoints and length

```
julia> linrange(0,pi,100)
0.0:0.03173325912716963:3.141592653589793
```
* Possibly relevant [mailing list discussion](https://groups.google.com/d/msg/julia-users/3hj7e4yJX60/P0ycagBDmuoJ)
Should use `zero((b-a)/(len-1))` instead of `zero(a)`.