Renamed tk to tc in erfa.h

This is no-op naming change to avoid confusion. The variables that represent temperature are all named `tc` in the *.c files, and are indeed temperatures in Celsius. In file `erfa.h`, the equivalent temperature parameters are named `tk`, which one could mistakenly interpret as temperature in Kelvin. In `erfa.h` I have therefore renamed all `tk` into `tc`.

Please check?