Fix reshape

[benedict-96]

Fixes #19
Fixes #18
Introduces a keyword argument reduce that specifies which operation should be used to perform mapreduce in build_nn_function (and similar functions).

But introduces type pyracy! Maybe Fixes #18 should be fixed by doing a pr in SymbolicUtils.

[codecov]

Codecov Report

Attention: Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.

Project coverage is 73.45%. Comparing base (8d610cc) to head (0ed585f).

Files with missing lines	Patch %	Lines
src/utils/build_function2.jl	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   72.30%   73.45%   +1.14%     
==========================================
  Files          16       17       +1     
  Lines         325      339      +14     
==========================================
+ Hits          235      249      +14     
  Misses         90       90              

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[benedict-96]

NOTE: This pr is set to "draft" until JuliaSymbolics/SymbolicUtils.jl#644 is fixed.