More accurate calculation of areafact in remapping (CICE-Consortium#849)

* Modified doc to specify that l_fixed_area is T for C-grid

* Initial modifs to calc areafact based on linear interpolation of left and rigth values

* put back l_fixed_area = .true. for C-grid

* added temporary comments for PR review

* Modified areafac calc for case 1 and case 2

* Corrected minor compilation issues

* Corrected conditions for case 1 to make sure areas add up

* Small modif in l_fixed_area section to ensure only one condition is true

* Modified conditions in locate triangle to be consistent with previous changes for case 1

* Use other edge areafac_c for TL, BL, TR and BR triangles

* Some comments removed

* Fixed out of bounds areafac_ce and now use earea and narea

* Replaced ib,ie,jb,je in locate_triangle using ilo,ihi,jlo,jhi

* Modified areafac for TL1, BL2, TR1 and BR2 for area flux consistency

* Cosmetic changes

* Added comment to explain latest change

* Modification of bugcheck condition for l_fixed_area=T

* update areafac_c, areafac_ce in halo in dynamics

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Co-authored-by: apcraig <[email protected]>