Fix interp2d import, issue #74

[rapoliveira]

The scipy function interp2d (to be deprecated soon, issue #74) was replaced by RegularGridInterpolator in two codes:

• souce/pointlens.py
• data/interpolate_elliptic_integral_3.py

I tested the changes only in the second code, which lead to differences smaller than 1e-10 in the interpolated values.

[rapoliveira]

@rpoleski the tests failed in test_Lee09_and_WittMao94() function of test_MagnificationCurve.py, but passed in my local repository. Since the failure is not related to my changes, should I try to commit once more?

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (2e55c55) 83.61% compared to head (8e879bb) 83.53%.
Report is 4 commits behind head on master.

Files	Patch %	Lines
source/MulensModel/pointlens.py	66.66%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
- Coverage   83.61%   83.53%   -0.08%     
==========================================
  Files          52       52              
  Lines        8271     8286      +15     
==========================================
+ Hits         6916     6922       +6     
- Misses       1355     1364       +9     

Flag	Coverage Δ
unittests	83.53% <66.66%> (-0.08%)	⬇️

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