Use stringstream to format default values

[WardBrian]

Submisison Checklist

• Run tests: ./runCmdStanTests.py src/test
• Declare copyright holder and open-source license: see below

Summary:

We use std::to_string to format default values (since #1172) but this does not print floating point numbers in scientific notation when required, see the example on cppreference comparing operator<< and std::to_string

Intended Effect:

Closes #1214

How to Verify:

./bernoulli help-all | grep -E "tol_(obj|grad|param)=<double>" -A 3

Side Effects:

Documentation:

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
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[mitzimorris]

lgtm!