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 :warning:

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Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/rpoleski/MulensModel/pull/113/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Radek+Poleski) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/rpoleski/MulensModel/pull/113/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Radek+Poleski) | `83.53% <66.66%> (-0.08%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Radek+Poleski#carryforward-flags-in-the-pull-request-comment) to find out more.

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