Added 2 new constructors for FileTape which copy the size of existing tape into a new one
Implemented external sort of a Tape using heap sort
Rewritten FileTape constructors implementation to better take advantage of existing constructor calls
Added const to some parameters to indicate their immutability
Added unit tests for external_sort
Disabled warnings as errors
bumped the cmake min version to 3.12 for c++20 support and -j flag
Placed a CMakeLists.txt in each subdirectory of the project
