Refactoring and reordering of internal header files

• Moved helper function such as unit_cast and unit_with_common_ratio to own file for easier maintenance
• Moved cross-unit-operations and epsEqual to a separate file for easier maintenance
• Rename epsEqual() to eps_equals() for consitency with naming scheme

Literals for `*ton_t`

• Add literals for ton types up to peta ton

`ton_t` for mass

• Add synonymous *_ton_t types for mass magnitudes
• Renamed default branch from master to main

All magnitudes for mass

• Documentation update and fixing typos in test names
• Full magnitudes for mass up to zetta_gram

Value-types for type traits

• Refactored type dependent functions to use std::enable_if_v instead of std::enable_if::value
• Use value types for type traits (i.e. std::is_arithmetic_v instead of std::is_arithmetic::value
• Add SI::detail::is_ratio_v and SI::detail::is_unit_t_v

Fractions as exponents

• Support for fractions as exponents (Types from version 1.x.x are no longer supported)
• Rename access to stored scalar values to value() (from previously raw_value()) (Brakes interfaces from 1.x.x)
• The conan-package is named si instead of SI to match the conan naming convention (Include-paths stay upper case)

Optimizing conan packaging

• Refactoring conanfile.py to create cmake object only once for a tiny bit more performance when packing

Buildsystem and packaging optimzations

• conanfile.py pulls the version out of CMakeLists.txt for packaging.
• Selecting the cmake option SI_INSTALL_LIBRARY=OFF removes the target install completely
• Add new-line to the end of all files to fit with the ConanCenter code convention.

Awareness about context when building with conan

• If SI is built from within conan the conanbuildinfo provided by conan will be used instead of getting it from github
• Cleanup of obsolete code

Speed of light as unit

• Extended technical documentation
• Add speed of light to velocity