Update imports, fixing #67

[matthewtownson]

• Updated imports to use numba rather than check for it - relating to infinitephasescreen unecessary handling of missing numba #67
• Updated imports to just use range (rather than xrange) as Python 2 has been deprecated

[matthewtownson]

Don't merge this! I have accidentally renamed the docs folder, and need to fix it first.

[matthewtownson]

OK, should be fine to merge now. I am not sure why the tests aren't running on the pull request...

[codecov]

Codecov Report

Merging #78 (d789da5) into master (1773d8d) will decrease coverage by 1.86%.
The diff coverage is 81.71%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   93.70%   91.83%   -1.87%     
==========================================
  Files          24       42      +18     
  Lines        1191     1996     +805     
==========================================
+ Hits         1116     1833     +717     
- Misses         75      163      +88     

Impacted Files	Coverage Δ
aotools/turbulence/__init__.py	100.00% <ø> (ø)
aotools/turbulence/atmos_conversions.py	100.00% <ø> (ø)
aotools/wfs/wfslib.py	100.00% <ø> (ø)
test/test_centroiders.py	100.00% <ø> (ø)
test/test_circle.py	100.00% <ø> (ø)
test/test_infinitephasescreen.py	46.15% <ø> (ø)
test/test_opticalprop.py	88.09% <ø> (ø)
test/test_phasescreen.py	100.00% <ø> (ø)
aotools/fouriertransform.py	74.07% <64.28%> (-2.85%)	⬇️
aotools/functions/zernike.py	96.80% <66.66%> (-0.10%)	⬇️
... and 26 more

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