fix for reading variables from xlsx file

[bwronska]

The function instance_from_excel returns the error while reading model variables from excel file. The following commit should fix that issue

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

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Files with missing lines	Patch %	Lines
mango/processing/file_functions.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #142   +/-   ##
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  Coverage           ?   76.11%           
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  Files              ?       73           
  Lines              ?     3399           
  Branches           ?        0           
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  Hits               ?     2587           
  Misses             ?      812           
  Partials           ?        0           

Flag	Coverage Δ
unit-tests	76.11% <0.00%> (?)

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