Added a node to keep track of files

[pfebrer]

This PR adds a FileNode that watches the filesystem for changes and gets marked as outdated if there is some change.

This is a step towards having auto-updatable plots, for example.

The way it works is that you simply initialize it with a path and then pass it to any argument that expects a path, and it will be like passing directly the path, except with the functionality of listening for updates.

from sisl.nodes import Node, FileNode
import time

# Write something to file
with open("testfile", "w") as f:
    f.write("HELLO")

# Create a FileNode
n = FileNode("testfile")

# Define a file reader node that prints the contents of the file
@Node.from_func
def print_contents(path):
    print("printing contents...")
    with open(path, "r") as f:
        print(f.read())

# And initialize it by passing the FileNode as an input
printer = print_contents(path=n)

print("---RUNNING NODE")

# Run the printer node
printer.get()

print("--- SET AUTOMATIC UPDATING")

# Now set it to automatically update on changes to upstream inputs
printer.context.update(lazy=False)

print("--- APPENDING TO FILE")

# Append to the file which will trigger the update.
with open("testfile", "a") as f:
    f.write("\nBYE")

# Give some time for the printer to react before exiting
time.sleep(1)

---RUNNING NODE
printing contents...
HELLO
--- SET AUTOMATIC UPDATING
--- APPENDING TO FILE
printing contents...
HELLO
BYE

• Added tests for new/changed functions?
• Ran isort . and black . at top-level

[zerothi]

Linting failed

[codecov]

Codecov Report

Attention: 48 lines in your changes are missing coverage. Please review.

Comparison is base (f7a3485) 87.73% compared to head (1d7229f) 87.65%.
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #647      +/-   ##
==========================================
- Coverage   87.73%   87.65%   -0.08%     
==========================================
  Files         362      364       +2     
  Lines       48436    48520      +84     
==========================================
+ Hits        42496    42532      +36     
- Misses       5940     5988      +48     

Files	Coverage Δ
src/sisl/nodes/__init__.py	100.00% <100.00%> (ø)
src/sisl/nodes/tests/test_file_nodes.py	58.82% <58.82%> (ø)
src/sisl/nodes/file_nodes.py	37.87% <37.87%> (ø)

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