Custom test harness

[FabrizioSandri]

Prior to this change, each time RcppDeepState was executed, a new folder was created in place of the existing inst/tesfiles directory, erasing its entire content. However, if the user wanted to provide a custom test harness, that file would have been removed alongside with inst/tesfiles.

To address this issue, I started this pull request to let the user manually create a test harness for a function for which there is no way to automatically generate the test harness(function's parameters are not within the permitted ones).

This pull request is related to this comment.

[FabrizioSandri]

Now the user has the possibility to provide a custom test harness inside the inst/testfiles folder and analyze it with RcppDeepState. This solves the problem that the test harness for binseg_interface was not run as described in this comment tdhock/binsegRcpp#13 (comment).

[FabrizioSandri]

In the last few commits I have solved issue #18 by forcing clang to use the -gdwarf-4 parameter.
In addition, I have updated the general code structure with commit 0b6cba1.

Fixes #18