Update comp_function() to take in separate inputs

[MayaSimone]

Found two methods of declaring multiple inputs in the functions of comp_function(), and have adjusted the test suite to only support one method. Going forward, each input must be declared as its own individual variable (not a single list to be iterated through). Updated docstrings still needed.

Ex.

gr.Model()
            >> gr.cp_function(
                fun=lambda a, b: a+b,
                var=["a", "b"],
                out=["c"],
            )

[zdelrosario]

Great! Like we expected, the laminate plate code is still failing. I think this should be an easy fix though; take a look at Line 412 of plane_laminate.py:

                Function(
                    lambda X: uniaxial_stress_limit(X),
                    make_names(Theta_nom),
                    list(itertools.chain.from_iterable([
                        ["g_11_tension_{}".format(i),
                         "g_22_tension_{}".format(i),
                         "g_11_compression_{}".format(i),
                         "g_22_compression_{}".format(i),
                         "g_12_shear_{}".format(i)] for i in range(k)
                    ])),
                    "limit states",
                    0
                )

We should be able to fix this by using a splat in the lambda function

                Function(
                    lambda *X: uniaxial_stress_limit(X),
                    make_names(Theta_nom),
                    list(itertools.chain.from_iterable([
                        ["g_11_tension_{}".format(i),
                         "g_22_tension_{}".format(i),
                         "g_11_compression_{}".format(i),
                         "g_22_compression_{}".format(i),
                         "g_12_shear_{}".format(i)] for i in range(k)
                    ])),
                    "limit states",
                    0
                )

This also suggests we can preserve old functionality if users define a lambda function using the * operator (which is something we can note in the docstring).

Can you give this edit a try and re-push?

[codecov]

Codecov Report

Patch coverage: 52.94% and project coverage change: -0.01 ⚠️

Comparison is base (8a65efb) 76.83% compared to head (c99bc1c) 76.83%.

❗ Current head c99bc1c differs from pull request most recent head af43395. Consider uploading reports for the commit af43395 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
- Coverage   76.83%   76.83%   -0.01%     
==========================================
  Files          87       87              
  Lines        6740     6739       -1     
==========================================
- Hits         5179     5178       -1     
  Misses       1561     1561              

Impacted Files	Coverage Δ
grama/eval_defaults.py	97.43% <ø> (ø)
grama/fit/fit_scikitlearn.py	77.41% <ø> (ø)
grama/models/circuit_RLC.py	100.00% <ø> (ø)
grama/models/plane_laminate.py	84.13% <ø> (ø)
grama/models/pipe_flow.py	52.17% <38.46%> (ø)
grama/core.py	84.61% <100.00%> (ø)
grama/models/ishigami.py	100.00% <100.00%> (ø)
grama/models/linear_normal.py	100.00% <100.00%> (ø)
grama/models/test.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[zdelrosario]

Great contribution! Some small changes necessary, and let's talk about updating the docstring for cp_function().