Add ability to see developer-defined 'name' and 'unit' fields

[richardbrodie]

No description provided.

[richardbrodie]

All field names and units are ordinarily defined in the SDK document, so don't need to be provided by this crate. But developer-defined fields are by definition not in the SDK so the user needs to know what the field is.

For example, currently a developer field is parsed to the following struct:

DevDataField {
    data_index: 1,
    field_num: 0,
    value: U8(87)
}

Which means basically nothing on its own. What is 87? Who knows?

The data provided in the developer definition, which we do not yet expose to the user, is as follows:

DeveloperFieldDescription {
    developer_data_index: 1,
    field_definition_number: 0,
    fit_base_type: 2,
    field_name: "charge",
    units: "%"
}

We can see that 87 is in fact 87% charge, which makes more sense.

We need some way to expose, at a minimum, "charge" and "%" to the user in some useful way.