Add .get_alt() method to Polyline class

[th3w4y]

Short Description:

Use Google Maps Elevation api to get the elevations along the path of the polyline

Fixes

• Implements .get_alt() method

[mention-bot]

@th3w4y, thanks for your PR! By analyzing the annotation information on this pull request, we identified @kanemasa1987, @sinap and @asnelzin to be potential reviewers

[MZorzy]

i think is better pick altitude at start.
then use some "noise" when walk, and upgrade altitude after x time, so low use google API (will not work after 2500 query?)

[th3w4y]

@MZorzy this uses only one API request per Polyline object created
With one request you get max 512 altitudes values along the polyline path (generated by one Directions API request)

Plus to mention we have 2500 Direction requests and 2500 Elevations requests, thus they consume a different quota since is a different API.

Thus this will not add any pressure on the quota of the already usage of the google API