fix: Fix ambiguous behavior of `extend` while adding a submodel

[kberat-synnada]

Closes #20

Description

Bug of extend fixed.

What is Changed

Ambiguity of model extension without calling submodel and calling submodel with no arguments bug fixed by calling submodel with no arguments in the beginning of extend if directly model is given.

Checklist:

• [x] Tests that cover the code added.
• [x] Corresponding changes documented.
• [x] All tests passed.
• [x] The code linted and styled (pre-commit run --all-files has passed).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.91%. Comparing base (39e3eb0) to head (3022b78). Report is 4 commits behind head on main.

@@           Coverage Diff           @@
##             main      #26   +/-   ##
=======================================
  Coverage   85.90%   85.91%           
=======================================
  Files          61       61           
  Lines       15249    15251    +2     
=======================================
+ Hits        13100    13103    +3     
+ Misses       2149     2148    -1     

Files with missing lines	Coverage Δ
mithril/framework/logical/model.py	96.22% <100.00%> (+0.01%)	:arrow_up:

... and 1 file with indirect coverage changes

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