Add darktable constants

[wpferguson]

darktable-org/darktable#14259 changes the representation of an invalid GPS coordinate from NaN to DT_INVALID_GPS_COORDINATE. DT_INVALID_GPS_COORDINATE is -FLT_MAX which is a C definition. This is all a part of replacing NaN use in darktable.

For a start I could create a constants library with the appropriate definitions and values that could be included as needed.

[wpferguson]

Depends on darktable-org/darktable#14259