ir2fr: easily calculate single-sided spectrum

* ir2fr to easily calculate single-sided spectrum
* neper_to_decibel and decibel_to_neper to convert between decibels and
nepers
Merge branch 'master' of github.com:python-acoustics/python-acoustics into ir2fr
Merge branch 'master' of github.com:python-acoustics/python-acoustics into ir2fr
Merge pull request #48 from FRidh/ir2fr

 ir2fr to easily calculate single-sided spectrum