Set a precision for the float numbers in the geojson file

[francocipollone]

Summary

Fix a precision number for the creation lat-lon coordinates that are dumped into the geojson file.
This will avoid different values when running in different machines. In particular it will unblock #55

Proposal

Using 7 decimals we are targeting a precision of 0.0001 m (111.139 * 1e-7 =~0.0001 )

Originally posted by @francocipollone in #55 (comment)

[liangfok]

I'm totally fine with the proposal since it's the simplest fix. Below are some random thoughts.

1. Auto-select the number of decimal places based on Maliput's inertial position tolerance. Choose the smallest number of decimal places to achieve a precision smaller than this tolerance.
2. Make the number of decimal places user-specifiable, with 7 decimal places as the default.