[FEATURE, TEST] Making find_deletions write to an output file and not to std::out. And adding tests.

[simonsasse]

Resolves 1/2 of #21

[Irallia]

Thanks for your work!
Could you please rebase on the current master? And connect the PR to the related issue (Button at the bottom of this page.). And write at least resolves #ISSUENUMBER in the description of this PR.
Most of my requests are more like discussion points.

[Irallia]

Wonderful, thank you very much!

[smehringer]

May I suggest something that does not need to be changed or part of this PR:
You go from functions foo() that print to std::cout to function foo(std::ofstream & out_file) that prints to a file. If you would do

template<typename stream_type>
void foo(stream_type & stream)
{
    stream << // ...
}

You can pass either a file OR std::cout. Writing to an dedicated output file is always very helpful in most application but I find sometimes, when writing pipelines, it can be very handy if the file can also be written to std::cout. This of course depends on your App and whether you think it may or may not be executed as part of a pipeline. If not you can ignore my suggestion, if so you might want to consider writing to cout unless a output_path is given. :) Just ideas

Note that there is also a output_stream seqan3 concept for checking that is has the << operator.