refactorization: coupler_clock object

[mlee03]

The derived type coupler_clock_type consists of (integer) clock ids and has been introduced as part of full coupler_main refactorization. Because of the frequency in which these ids are retrieved in coupler_main, all components of coupler_clock_type have initially been made public.

However, to be consistent with current FMS developments to objectify all derived types - making derived types components private and introducing "getters" and "setter" type-bound procedures, the following changes should be made:

1. All ids should be made private.
2. Type-bound functions to retrieve clock ids should be introduced.
3. Existing clock-related subroutine coupler_set_clock_ids should be made into a type-bound subroutine.

[mlee03]

@thomas-robinson @abrooks1085