Fixes for parallell runs

[oskbor]

• Create markers/{slot_name} early in run_marker_search so other processes can detect that marker search is in progress for this slot.
• Add try in __get_size for Metadata. This mitigates issues that can happen when another process is currently writing a zarr group to the hierarchy.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 67.44%. Comparing base (440bab1) to head (c00d268).

Files	Patch %	Lines
scarf/metadata.py	50.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
- Coverage   67.46%   67.44%   -0.01%     
==========================================
  Files          22       22              
  Lines        5577     5580       +3     
==========================================
+ Hits         3762     3763       +1     
- Misses       1815     1817       +2     

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