Deprecate dsp.fractional octave frequencies

[f-brinkmann]

Which issue(s) are closed by this pull request?

Deprecate dsp.fractional octave frequencies in favour of constants.fractional octave frequencies_nominal and constants.fractional octave frequencies_exact

Changes proposed in this pull request:

• add and test deprecation warning to dsp.fractional octave frequencies
• replace use of dsp.fractional octave frequencies everywhere

Tests fail because of #857 and should be fixed there. To fix them it would be required to change

• test_fractional_octaves, line 76 to sr, 1, frequency_range=(10e3, 20e3), order=order) and
• test_fractional_octaves, line 193 to frequency_range = (30, 20000)