Fixes precision loss when adding nested models.

[hauke76]

The precision loss is resolved by explicitly calling std::setprecision
for float and double parameters when these are streamed/requested as
strings.

[scpeters]

addresses #313

[scpeters]

I think we would need a test before we could merge this. Fortunately, there are some existing tests for the text output after including nested models (see test/integration/nested_model.cc) that could be modified to increase precision. For example, scpeters@dce1c4d increases the precision of a pose value in a nested model and expects the same precision in the output. Is that the type of thing this is supposed to fix? I tried that test with your branch, and it didn't seem to fix it. With which data types have you tested your branch? I think Vector3 and Quaternion values might have precision limited by the ign-math class:

• https://github.com/ignitionrobotics/ign-math/blob/master/include/ignition/math/Quaternion.hh#L1208-L1219
• https://github.com/ignitionrobotics/ign-math/blob/master/include/ignition/math/Vector3.hh#L737-L747

[hauke76]

The proposed fix only addresses double and float types stored in the parameter variant. I ran into this when using camera intrinsics such as fx, fy, etc. I already thought about the vector and matrix types. We need special branches to fix that too as the default streaming operators you pointed to only serialize with 6 decimals precision.

I will extend this CL to also fix those types and add a test.

[hauke76]

I have modified the existing test to show that the correct code path is taken and to show that the logic is correct. Since I have not found an SDF property with 32bit float precision, I only tested for double precision. I used the sphere's radius property which is a double.

All floating point ignition math types are affected as well. I could easily provide full specializations for them as and re-implement the serialization while taking care to exactly copy the math libraries' behavior but I am wondering if the better fix were to remove the precision function invocation in the ignition math library. It were then possible to simply write os << std::setprecision(std::numeric_limits<T>::max_digits10) << v.val for the ignition types. Right now, that won't work.

[EricCousineau-TRI]

For the std::setprecision(), would it be possible to make sure you reverse any global effects? e.g. using some RAII pattern?

[EricCousineau-TRI]

Relates #252 / gazebosim/sdf_tutorials#37. For example:
https://github.com/osrf/sdf_tutorials/blob/921244bca7068050d5d2c74b9dec58bf0f46ddb3/specify_pose/proposal.md#motivation

[jennuine]

For the std::setprecision(), would it be possible to make sure you reverse any global effects?

There are no global effects because the ostream used in the operator<< functions is a locally created stream object

[EricCousineau-TRI]

Sweet, thank you for checking!