[WIP] deconstructPathRelativeToSWD

Addresses fixes to deconstructPathname (addresses issue #264). Introduces a new function deconstructPathRelativeToSWD that finds the absolute path to a given path name relative to a specified working directory (swd).
> 1. What do we even want a non-Windows machine to return if a path has a drive letter?

So I just checked, and on linux I can create a directory like `/home/chris/C:/hello`. So I think a drive letter on windows is treated just like a directory name, since that's possible (though, why would someone??). That is, on non-windows, treat `C:/arbitrary/path` as a relative path, just as you'd treat `hello/world`.

> 2. How to do tests where we might/would expect different answers on different platforms?

Maybe separate the tests by platform?

```cpp
#ifdef _WIN32
    windows path tests
#else
    other path tests
#endif
```

See [this code example](https://github.com/simbody/simbody/blob/48c3cc037e47c42770bf2e0f708814976bf8a739/SimTKcommon/tests/TestPlugin.cpp#L135).

If we wanted to account for the potential for people to cross-compile simbody (compile on windows for release on unix), then stuff gets really complicated.