Pathlib + f-strings

[ecomodeller]

Pathlib is an object oriented way of handling files and paths (Python 3.4+)

f-strings is string interpolation, more readable than concatenating strings (similar to C# https://learn.microsoft.com/en-us/dotnet/csharp/tutorials/string-interpolation) (Python 3.6+)

This PR is not a complete conversion of all files, only engine1d.py. Use it as inspiration.

[gedaskir]

@ecomodeller I think the current change makes the string/path manipulations look more readable. Regarding the use of os.path vs. pathlib.Path I think it is a matter of preference and both can be used equally well, especially in the case if the old code everywhere used os.path. Though, I also prefer pathlib. It is good to avoid statements like file_name = file.split('.')[0] and if using os.path it could be written as name, extension = os.path.splitext('/path/to/somefile.ext'). Regarding the + operator for strings, even PEP8 has something to say about it (though performance consideration likely here is not an issue):

For example, do not rely on CPython’s efficient implementation of in-place string concatenation for statements in the form a += b or a = a + b. This optimization is fragile even in CPython (it only works for some types) and isn’t present at all in implementations that don’t use refcounting. In performance sensitive parts of the library, the ''.join() form should be used instead. This will ensure that concatenation occurs in linear time across various implementations.