Bring some utility functions to fsspec.FUNC

[hadim]

There are plenty of small utility functions that could be applied from a URI without having to instantiate a filespec object. It makes life much easier when you want to deal with different types of filesystem transparently.

Here are two examples:

def is_valid_file(path):
    """Check whether a file is a valid path"""
    mapper = fsspec.get_mapper(path)
    return mapper.fs.isfile(path)


def is_local_path(path):
    """Check whether a path point to a local filesystem."""
    mapper = fsspec.get_mapper(path)
    return mapper.fs.protocol == "file"

I guess the idea is similar to what fsspec.open already provides.

[martindurant]

This is essentially the same idea as #41
But I do welcome the addition of utility functions in general.