Add a check for duplicate neighbouring cells within the theory pair counters (periodic)

[manodeep]

Is your feature request related to a problem? Please describe.
When periodic boundary conditions are enabled, Corrfunc should be able to calculate separations out to the maximum possible separation. Instead, Corrfunc might fail with the following error message

*Error message:
../../utils/gridlink_utils_float.c> ERROR: nlattice = 4 is so small that with periodic wrapping the same cells will be counted twice ....exiting
../../utils/gridlink_utils_float.c> Please reduce Rmax = 100.000000 to be a smaller fraction of the particle distribution region = 205.000000

Describe the solution you'd like
While adding a new neighbouring cell, add a check for duplicate neighbouring cells. A similar check is already performed here. Easiest option would be to package those lines of code into a function within gridlink_utils.c.src and call that function from all relevant assign_ngb calls.

Describe alternatives you've considered
N/A

Additional context
Reported by @JonLoveday in #69

[manodeep]

@JonLoveday I pushed a fix via e53bce3. Should not be too long before we can merge this into master.

If possible, would you please checkout the code_quality branch and see if the issue is resolved?

[manodeep]

@JonLoveday The fix is now on the master branch - I am closing this issue.

If the fix does not solve your case, please feel free to re-open this issue.

[manodeep]

This is not fixed - see #210

[manodeep]

This is done / superseded by #277