Fix handling of overscan regions with darks/flats.
Store header keys in a hashmap.
Tweak hashmap to use const keys.
Use hashmap for key queries.
Close input fits file immediately after loading.
Make framedata processing return an error code.
Make framedata_start_time return an error code.
Remove obsolete die() function.
Support subsecond resolution for darks and flats.
Add new metadata lookup functions.
Use new metadata lookup functions everywhere.
Remove old metadata lookup functions.
Merge alloc/load and rename fp -> fd.
Remove unnecessary #include in aperture.c
Use calloc instead of malloc/zeroing in aperture.c.
Add a function for removing metadata keys.
Add a function for adding metadata.
Add a debug function for listing all header keys.
Also store metadata as a linked list to remember order.
Add 'tsreduce metadata' for printing metadata.