Fix for MAGN-3916: Straight, non-Line argument to ModelCurve.ByCurve causes failure

### Issue

All ModelCurve's require a host SketchPlane in order to exist.  For non-straight curves, ModelCurve.ByCurve determines this plane using a best fit plane.  A Line argument is a special case and was previously correctly handled. 

Straight, non-Line arguments (e.g. a NURBS curve with aligned control points) were NOT correctly handled and would cause the best fit plane code to be invoke.  The best fit plane for a straight line is non-determinate so this code was failing.

### Fix

The fix was to do a check before hand to determine if the curve was straight, even if it's not a Line.  This is done by tesselating the curve, forming a line between the end points, and looking for any tessellation point that is not along this line.  

This code is located in a new utility class called `CurveUtils` in the `Revit.GeometryConversion` namespace in RevitNodes.  The code for determining the plane from a Revit Curve was also moved here so that it can be reused more easily.


### Unit tests

Before discovering the source of this issue, I added several unit tests for `ProtoToRevitCurve.ToProtoType(NurbsCurve)`.  I've also added unit tests for `ModelCurve.ByCurve` and `CurveUtils.IsLineLike` testing the above functionality.

@ikeough.  
You can take a look at the ModelCurve code to get an idea of how this works.  It still makes sense to do this however, as we want to have a as close a match to the plane as possible.  
![3dcurve](https://cloud.githubusercontent.com/assets/916345/3705411/a89eb95a-141e-11e4-8083-f8af89e4fd3a.png)