Removing voigt_old functionality

[chummels]

Trident has historically had two separate methods for numerically integrating the voigt profiles: one from some old IDL routines using chebyshev polonomials and one using a SciPy method. If you have scipy installed it would use the scipy method, otherwise it would use the IDL routine. At some point, the IDL routine stopped working. This PR removes that functionality and makes SciPy a hard dependency of Trident.

Note: We have a test to ensure that the two methods get the same values for a set of inputs in test_absorption_spectrum.py, which never seemed to fail. I want to investigate this first before merging to find out what went awry here.

[coveralls]

Pull Request Test Coverage Report for Build 1093

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.09%) to 75.824%

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Files with Coverage Reduction	New Missed Lines	%
/home/circleci/trident/trident/absorption_spectrum/absorption_line.py	3	89.66%

Totals
Change from base Build 1066:	-0.09%
Covered Lines:	1772
Relevant Lines:	2337

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💛 - Coveralls

[brittonsmith]

Did you still want to investigate why the test was able to pass all this time? If not, I'm fine for this to be merged.

[brittonsmith]

Seems like this test was meant to compare the two functions. We probably don't need to keep it.

[brittonsmith]

RIP voigt_old. You were a good function.