Consistently normalize paths

[leoschwarz]

Fixes #76

Input paths were normalized inconsistently before. This will be fixed by this PR and will also yield better error messages if a similar problem reoccurs.

As a result the simple Bash-style leading tilde expansion pattern is supported now (which was the problem in #76, not relative paths in general), more complex forms are not (e.g. ~user or any other more advanced ones as these are very shell specific).

Note that these pattern features are widely unsupported or inconsistently supported by many tools and libraries, for instance the md5sum tool from GNU Coreutils doesn't understand the tilde either:

[tobiasko]

nice! Yes, you are right, this is a tilde expansion. But would a ../sample.raw now also work?

[leoschwarz]

Yes, while normalizePath does not convert such a relative path into an absolute path, it is supported. e.g. I tested rawr::readFileHeader with ../extdata/sample.raw (on macOS; but it should work on Windows too).