Add a mode param in Proxy class

[FrancescAlted]

Then pass that to the empty/SChunk constructor. This would avoid blosc2.remove_urlpath() calls in tests and examples.

The signature of the Proxy constructor would become something like:

def __init__(self, src: ProxySource or ProxyNDSource, urlpath: str = None, mode = "a", **kwargs: dict):

Remember to add the docstring in the constructor.

After that, most of the calls to blosc2.remove_urlpath() previous to Proxy() calls in tests and examples can go away and be replaced by Proxy(urlpath=..., mode="w", ...).