Fix warnings in Version3_1 unit tests

[rapoliveira]

I applied two recent changes from master branch to the Version3_1 branch, in order to solve warnings in the unit tests. The first change is related to interp2d replacing by RGI (issue #74, PR #113) and the second is about NumPy deprecation warnings related to converting length-1 arrays into floats (issue #127, PR #130).

Previously, there were 89 warnings and now there are only 9. The missing warnings seems to have been inserted on purpose, so I kept them.

The following two tests were failing with TypeError:

FAILED source/MulensModel/tests/test_Model.py::TestBLPS02AC::test_mag_calculation_1 - TypeError: only length-1 arrays can be converted to Python scalars
FAILED source/MulensModel/tests/test_Model.py::TestBLPS02AC::test_mag_calculation_2 - TypeError: only length-1 arrays can be converted to Python scalars

and now only the first one is failing with AssertionError:
FAILED tests/test_Model.py::TestBLPS02AC::test_mag_calculation_1 - AssertionError:

[rpoleski]

For simplicity, I've committed the changes directly to Version3_1 branch, because only some of this changes were incorporated:

• RGI - see 06635a7,
• float() - this will be done separately,
• adding [0] in 3 cases of 1-values arrays: 6a47319.