FEAT: +TOKEN modifier for side-effect functions

[ankostis]

Inputs & outputs in needs & provides kwargs designated as token work as usual while solving the DAG but they are never assigned any values to/from the operation's functions:

from graphkit import operation, compose
from graphkit.modifiers import token

# Function appending a new dataframe column from two pre-existing ones.
def addcolumns(df):
    df['sum'] = df['a'] + df['b']

# Designate `a`, `b` & `sum` column names as an token arguments.
graph = compose('mygraph')(
    operation(
        name='addcolumns',
        needs=['df', token('a'), token('b')],
        provides=[token('sum')])(addcolumns)
)

# The graph works with and without 'c' provided as input.
df = pd.DataFrame({'a': [5], 'b': [2]})
assert graph({'df': df})['sum'] == 11

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[ankostis]

Just didi it.

Is it understood what I'm trying to achieve?

[codecov-io]

Codecov Report

Merging #15 into master will increase coverage by 13.14%.
The diff coverage is 92.81%.

@@             Coverage Diff             @@
##           master      #15       +/-   ##
===========================================
+ Coverage   77.87%   91.01%   +13.14%     
===========================================
  Files           5        6        +1     
  Lines         348      501      +153     
===========================================
+ Hits          271      456      +185     
+ Misses         77       45       -32

Impacted Files	Coverage Δ
graphkit/modifiers.py	100% <100%> (ø)	⬆️
graphkit/functional.py	93.9% <100%> (+0.15%)	⬆️
graphkit/plot.py	89.68% <89.68%> (ø)
graphkit/base.py	81.01% <90.9%> (+1.64%)	⬆️
graphkit/network.py	94.14% <94.11%> (+23.94%)	⬆️

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