Add .gitattributes file

[phil-blain]

PR checklist

• Short (1 sentence) summary of your PR:
  Add a .gitattributes file at the base of the repo
• Developer(s):
  Philippe Blain
• Suggest PR reviewers from list in the column to the right. @apcraig
• Please copy the PR test results link or provide a summary of testing completed below.
  No tests needed
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • Yes
  • No
    I will do the same for Icepack if this is accepted.
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please provide any additional information or relevant details below:

This makes 'git diff' more useful by showing in which subroutine the shown changes are in the hunk header :
without .gitattributes (check the hunk header @@ ... @@):

$ git  diff
diff --git a/cicecore/cicedynB/dynamics/ice_dyn_vp.F90 b/cicecore/cicedynB/dynamics/ice_dyn_vp.F90
index 5533315..7e3b810 100644
--- a/cicecore/cicedynB/dynamics/ice_dyn_vp.F90
+++ b/cicecore/cicedynB/dynamics/ice_dyn_vp.F90
@@ -229,7 +229,10 @@
       type (block) :: &
          this_block           ! block information for current block
       
-      real (kind=dbl_kind), allocatable :: bvec(:), sol(:), diagvec(:)
+      real (kind=dbl_kind), allocatable :: &
+         bvec(:)     , & ! right-hand-side vector
+         sol(:),     , & ! solution vector
+         diagvec(:)      ! diagonal vector
       
       character(len=*), parameter :: subname = '(imp_solver)

with .gitattributes (check the hunk header @@ ... @@):

$ git  diff
diff --git a/cicecore/cicedynB/dynamics/ice_dyn_vp.F90 b/cicecore/cicedynB/dynamics/ice_dyn_vp.F90
index 5533315..7e3b810 100644
--- a/cicecore/cicedynB/dynamics/ice_dyn_vp.F90
+++ b/cicecore/cicedynB/dynamics/ice_dyn_vp.F90
@@ -229,7 +229,10 @@ subroutine imp_solver (dt)
       type (block) :: &
          this_block           ! block information for current block
       
-      real (kind=dbl_kind), allocatable :: bvec(:), sol(:), diagvec(:)
+      real (kind=dbl_kind), allocatable :: &
+         bvec(:)     , & ! right-hand-side vector
+         sol(:),     , & ! solution vector
+         diagvec(:)      ! diagonal vector
       
       character(len=*), parameter :: subname = '(imp_solver)'

This can also be done locally in the file .git/info/attributes but I think it's useful enough to put it in the repo.