Changes to _eval_step_func

[kombatEldridge]

In my project, I am wanting to use some step functions while Meep runs. Currently, my entire simulation of Meep runs in a Simulation class and I would like to keep my step functions in this class as well. That being said, they must have self as an arg. With the current _eval_step_func, the arg handler is a bit too strict. There is probably a reason for this, and please disregard this PR if so.

The changes I am requesting would include the possibility that the step functions contain an arg named self but should still keep the intended limitations of the previous version.

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Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 73.71%. Comparing base (f29a8c7) to head (566c47d).
Report is 33 commits behind head on master.

Files with missing lines	Patch %	Lines
python/simulation.py	83.33%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2895      +/-   ##
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- Coverage   73.81%   73.71%   -0.10%     
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  Files          18       18              
  Lines        5423     5449      +26     
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+ Hits         4003     4017      +14     
- Misses       1420     1432      +12     

Files with missing lines	Coverage Δ
python/simulation.py	77.41% <83.33%> (+<0.01%)	⬆️

[oskooi]

Currently, my entire simulation of Meep runs in a Simulation class and I would like to keep my step functions in this class as well. That being said, they must have self as an arg. With the current _eval_step_func, the arg handler is a bit too strict.

If you haven't already done so, it might be useful to first read The Run Function is not a Loop which provides a description of the step-function object including examples.

[stevengj]

Note that you can already pass as many additional arguments you want to the step function by wrapping it in a closure.

For example, if you have a function my_step(a,b,c,sim), you can pass it as a step function with lambda sim: my_step(a,b,c,sim), in which the additional arguments a,b,c are captured by the closure.

Or, if you have a function foo.my_step(sim) that is a bound method of some object foo, you can pass it as a step function with lambda sim: foo.my_step(sim).

[stevengj]

Overall, I'm in favor of something like this to make it easier to pass bound methods for step functions.

However, what I'd like to do is to simply fix get_num_args to subtract 1 if the function is a bound method, as determined by inspect.ismethod(func) (not from the argument names). That is, get_num_args should return the number of non-self arguments.