`Estimator.L` can be `Estimator.src.L`

[chris-langfield]

Followup to the discussion on #517

The Estimator class derives its L property directly from the Source from which it is instantiated.

ASPIRE-Python/src/aspire/reconstruction/estimator.py

Line 24 in 3223d6c

self.L = src.L

Similarly NoiseEstimator https://github.com/ComputationalCryoEM/ASPIRE-Python/blob/master/src/aspire/noise/noise.py:

        self.src = src                                                                        
        self.dtype = self.src.dtype                                                           
        self.L = src.L                                                                        
        self.n = src.n   

We should double check that these aren't ever different, and if so, simplify by referring to the src if that's where the values come from.

[chris-langfield]

Closed by #550