Additional changes in KPP scheme

[alperaltuntas]

• clean up unnecessary code in KPP_calculate. all the removed code was already copied into KPP_compute_BLD previously in PR Add KPP multi-smoothing and deepening-only smoothing options, and viscosities due to tidal mixing #65.
• fix boundary layer depth limiter after smoothing.
• rename MOM_KPP.F90 as MOM_CVMix_KPP.F90

[gustavo-marques]

This PR fails the following test:
MOM6-examples/ocean_only/CVmix_SCM_tests/mech_only/KPP

MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: forrtl: severe (151): allocatable array is already allocated
MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: Image              PC                Routine            Line        Source             
MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: MOM6               000000000157F0EC  Unknown               Unknown  Unknown
MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: MOM6               0000000000B4B3D9  mom_kpp_mp_kpp_in         530  MOM_CVMix_KPP.F90
MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: MOM6               000000000066635A  mom_diabatic_driv        3088  MOM_diabatic_driver.F90
MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: MOM6               0000000000A07BBB  mom_mp_initialize        2323  MOM.F90
MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: MOM6               0000000000C0A8D9  MAIN__                    309  MOM_driver.F90
MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: MOM6               0000000000404F9E  Unknown               Unknown  Unknown
MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: libc-2.19.so       00002AAAAB832B25  __libc_start_main     Unknown  Unknown
MOM6-examples/ocean_only/CVmix_SCM_tests/cooling_only/KPP/ocean.stats.intel: MOM6               0000000000404EA9  Unknown               Unknown  Unknown

Please delete lines 530-531 in MOM_CVMix_KPP.F90, since CS%N is already been allocated/initialized in lines 509-510.

[gustavo-marques]

This PR does not change answers for MOM6-examples (dev-master-candidate-2018-05-15)