Fix memory leak in emp::DataFile

[FergusonAJ]

Bug description
One of the constructors in emp::DataFile doesn't take an std::ostream, instead it takes a filename string.
It then uses that string to create a new std::ostream pointer. However, this pointer (os) is never deleted. Since it's not an Empirical pointer, this is not detected in debug mode.

Relevant code (line 52 at time of writing):

    DataFile(const std::string & in_filename,
             const std::string & b="", const std::string & s=",", const std::string & e="\n")
      : filename(in_filename), os(
      #ifdef __EMSCRIPTEN__
      new emp::NullStream()
      #else
      new std::ofstream(in_filename)
      #endif
      ), funs(), pre_funs(), keys(), descs()
      , timing_fun([](size_t){return true;})
      , line_begin(b), line_spacer(s), line_end(e) { ; }

To Reproduce
The DataFile example (examples/data/DataFile.cpp) has this issue without any modifications.
Here's the valgrind command I used to show the issue:

valgrind --leak-check=full --show-reachable=yes --leak-resolution=high --num-callers=100 --trace-children=yes ./DataFile 

Expected behavior
Removing the memory leak is the main priority.
Switching the os pointer to be an emp::Ptr would improve consistency with rest of codebase.
Destructor and copy/move constructors will likely need to be updated to accommodate issue (rule of five).

Toolchain (please complete the following information):

• OS: Pop!_OS 21.04
• Compiler: gcc version 10.3.0 (Ubuntu 10.3.0-1ubuntu1)
• Empirical Version master at 1fe7f90