Splines Type

Nobody has claimed this yet.

We have a preliminary working set of vector tiles being created with a new data type that includes splines. The type of splines being used are b-splines. The control points of the spline are encoded in the same way a linestring would be encoded, with the knots being added to a new field of a repeated double in the Feature message.

Currently, the Spline is denoted by a GeomType
```
enum GeomType {
UNKNOWN = 0;
POINT = 1;
LINESTRING = 2;
POLYGON = 3;
SPLINE = 4;
}
```
and knots for the Feature message being added to field 5:
```
repeated double knots = 5 [ packed = true ];
```
An example implementation of this is located [here](https://github.com/mapbox/vector-tile-base/blob/master/vector_tile.proto).

## Pros

* This is a complete way to represent a curve
* Very precise curved structures
* Explicit type (compared to using a CurveTo command with existing types)
* Could be interpolated into linestring in decoders if necessary

## Cons

* Splines are complex and could be difficult for some people to utilize properly
* Does not provide a clear curve for polygon types
* Increased complexity of types

Start by reviewing the current vector tile specification and the example implementation in vector_tile.proto. Read the seven comments to understand the unresolved tradeoffs around SPLINE, b-spline control points, and the knots field. Done means the proposal has a documented decision and, if accepted, the specification reflects the agreed representation.

Written by the indexing model from the issue text.