FR: allow lat/lng str instead of float

**Is your feature request related to a problem? Please describe.**
A multi-location elevation() request throws an err when the locs are a list of lat/lng strings.

**Describe the solution you'd like**
I would have expected this to work.
```
locations = [('40.714728', '-73.998672'), ('-34.397', '150.644')]
results = client.elevation(locations)
```
Since I'm retrieving them from json text and googlemaps eventually returns them as json text, why require manual/interim conversion to floats?

**Additional context**
It appears this might be due to the polyline encoding. But still...why is that mandatory for small lists of coords?