Download helper function

[pllim]

Fix #178

TODO:

• Write tests
• Test on Linux
• Test on Windows (Appveyor)
• Update docs to get rid of FTP instructions and use this function instead

[codecov]

Codecov Report

Merging #179 into master will decrease coverage by 0.06%.
The diff coverage is 92.59%.

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
- Coverage      97%   96.93%   -0.07%     
==========================================
  Files          13       13              
  Lines        1705     1732      +27     
==========================================
+ Hits         1654     1679      +25     
- Misses         51       53       +2

Impacted Files	Coverage Δ
synphot/utils.py	97.91% <92.59%> (-2.09%)	⬇️

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