[Tensor] Fix invalid getIdx() method

Context

The Tensor getIdx() method for computing flat index from a coords list is wrong. There were two issues:

• when the coords is 0-rank (i.e. coords == {}) it created a segfault due to an access to an out-of-bound access
• when the coords rank differed from the tensor rank by more than 2 dimensions the computation was incorrect

This MR fixes getIdx() and add unit tests for get()/set()/getIdx() with index or coords.

After adding the first commit (unit tests) here is the output of the unit tests:

unit_testsbuild/unit_tests/tests_aidge_core --rng-seed 0 -c 'Tensor set/get'
/data/Test_Tensor.cpp:337: FAILED:
  {Unknown expression after the reported line}
due to a fatal error condition:
  SIGSEGV - Segmentation violation signal

===============================================================================
test cases:   5 |   4 passed | 1 failed
assertions: 232 | 231 passed | 1 failed

Segmentation fault

After applying the second patch which fixes the issue, here is the output:

unit_testsbuild/unit_tests/tests_aidge_core --rng-seed 0 -c 'Tensor set/get'
===============================================================================
All tests passed (8009 assertions in 77 test cases)

Modified files

First commit: add unit tests Tensor set/get to Tensor unit tests (note that with this commit in isolation the unit tests fail. Second Commit: fix implementation of getIdx() Third commit: minor change to getStorageIdx() which did not assert dimensions size before checking coords values.

Detailed major modifications

No major modification.

TODO

No forecasted todo.