sdp::grammar::number_converter precision

[garethsb]

The sdp::grammar::number_converter currently uses sufficient precision to guarantee roundtrip of all double values (by virtue of using the web::json::value::serialize() function, with the particular effect that a number like 59.94 is serialized as 59.939999999999998. Good for test cases, but probably not much else. See microsoft/cpprestsdk#706.

A custom attribute converter can be used to work around this issue, though the workaround ends up a little more verbose than one would like.

Some way of globally (within one make_session_description call) specifying precision or the number format in general was previously considered and rejected.

Another option may be to follow the resolution taken for nlohmann/json#360 and adopt the Grisu2 algorithm from Florian Loitsch. Since nmos-cpp already has nlohmann/json as a dependency, its implementation could be used.