add low_memory option

[lmiq]

This PR adds the low_memory option to the mddf function which, set to true:

• Defines the number of chunks to use at most 20% of the total memory of the system,
  considering the size of the results data structure.
• Parallelizes the execution of CellListMap to use nthreads/nchunks number of threads.

[codecov]

Codecov Report

Attention: Patch coverage is 94.79769% with 9 lines in your changes missing coverage. Please review.

Project coverage is 86.68%. Comparing base (31f7406) to head (4bc63c6).

Files	Patch %	Lines
src/mddf.jl	94.11%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
+ Coverage   85.01%   86.68%   +1.67%     
==========================================
  Files          23       24       +1     
  Lines        1248     1367     +119     
==========================================
+ Hits         1061     1185     +124     
+ Misses        187      182       -5     

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