Allow multiple input files in tapac

[vkomenda]

Currently tapac only allows one input file:

positional arguments:
  file                  Input file, usually TAPA C++ source code.

whereas compilers like GCC can take multiple.

It's not critical since any project containing multiple files can be reduced to a single file by concatenation. However adding a multiple source file capability would make the tool more convenient.

[Blaok]

@dotkrnl has done an awesome refactoring that makes it possible to support multi input files. It is still under tested at this point but please do stay tuned :)

[dotkrnl]

Currently we only allow multiple files by using #include. It does allow users to write TAPA tasks in a separate file though. This is not yet implemented in TAPA as of now. Concatenation does work for simple projects but not ones with namespace conflicts. We do not have a proposal of how to deal with this. Helps are welcome!