Checks the specified filepath for formatting consistencies:
1) use slash instead of backslash;
2) do tilde etc. expansion;
3) remove trailing slash.
Usage
normPath(path)
## S3 method for class 'character':
normPath(path)
## S3 method for class 'list':
normPath(path)
## S3 method for class '`NULL`':
normPath(path)
## S3 method for class 'missing':
normPath()