UPD: remove 'iostream' and include only what is used

Context

The goal of this MR is three fold:

• Removing unused libraries that can lead to longer compilation time and bigger code
• Including everything that is used following https://google.github.io/styleguide/cppguide.html#Include_What_You_Use and https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes principles
• Removing <iostream>, <sstream> and <ostream> that are big includes and slow to run for the new faster fmt library that allows more formating options instead https://github.com/fmtlib/fmt

Changes

• CMakeLists.txt get latest fmt version if not found on system and change lib propagation rule to PUBLIC
• unit_tests/CMakeLists.txt add info if Catch2 found on system for core unit-tests
• pybind_Tensor.cpp add backend() binding
• ReduceMean.hpp add missing default values in constructor
• replace stream usage with fmt everywhere and include only what is used
• add backend-agnostic backward() implementation to Reshape operator

Change in tests_aidge_core binary size

As a result of this MR changes, the aidge_core test binary went from 5'899'032 bytes to 5'902'368, so a slight increase but it was no the binary targeted by these changes.