Handle missing elevation data

Heightgraph currently doesn't handle cases where part or all of the data points have coordinates, but no elevation/altitude value.

This results in incomplete graphs and NaN errors/warnings, see comparison below and the Plunker console:

current | expected
-- | --
[Plunker](http://plnkr.co/edit/BWSX4Y9XjCdyLCiy?preview) | [Plunker](http://plnkr.co/edit/0jLCagjFvf2o0HNZd09M?p=preview), see also [Elevation#84](https://github.com/MrMufflon/Leaflet.Elevation/pull/84)
 | 

Diagrams shown are variants of the same elevation profile with missing values at different positions:
1. original
2. middle (left of second segment)
3. left
4. right
5. all missing

Reasons for missing elevation data e.g. are using CGIAR CSI SRTM that has no data beyond 60 degrees north and south and outside shorelines, see abrensch/brouter#137, nrenner/brouter-web#203.

The suggested solution would be to show missing elevation values as gaps by using a d3 [defined](https://github.com/d3/d3-shape/blob/master/README.md#area_defined) accessor function, e.g. [here](https://github.com/GIScience/Leaflet.Heightgraph/blob/9e45353ae01e4389124701bdacb62ee255d3fec1/src/L.Control.Heightgraph.js#L776-L780):
```
d3Area().defined(d => d.altitude !== undefined && d.altitude !== null)
```

I guess I could provide a PR if no one else feels like it.