Merge filename path in results objects

[SteSeg]

Minor change in the source code. The two main classes (ResultsFromDatabase and ResultsFromOpenmc) used to take as arguments in the constructor the filename and path strings and then generate the path to the file by merging the two strings. E.g.:

ofb.ResultsFromDatabase(filename='myfile', path='path/to/')

Now they take in just one string with the full relative path already:

ofb.ResultsFromDatabase(file='path/to/myfile')

Tutorial notebooks and models have been update and tested accordingly.