Add mechanism to raise warnings #27
Description
Add a way to raise warnings using coda_report_warning() and coda_set_warning_handler() (similar to the way it is done in HARP).
The idea is to use this when CODA opens a product using a self-describing data format to raise warnings for content that is not supported by CODA. A warning handler will be set for codacheck by default to report on these warnings. For other tools like codadump this could potentially become a command line option.