Add support for reading developer data fields

[ryangay]

Hi,

This pull request adds rudimentary support for parsing developer data fields.

Overview of the changes that I have made:

• field_num becomes a FieldNum enum instead of a u8
• During parsing, developer data field descriptions are collected in a HashMap that maps a u8 tuple, representing the developer data index and definition number, respectively, to a new FieldDefinition struct containing them values plus the FIT base type of the developer data field
• The developer data field descriptions map is passed to parse_data_field to correctly parse developer data fields
• Values that represented 'field nums' are updated to be values of the FieldNum::GlobalProfile variant
• Fixed a timestamp out of range bug; however I don't know if this is functionally correct

Please let me know what you think!

Thanks,
Ryan