Fixing _higuchi_fd

[jbschiratti]

fixes #53 fixes the function _higuchi_fd:

• fix signatures in @nb.jit decorator
• kmax parameter should be of type np.int64 before lk = np.empty((kmax,)) is called unless Numba fails to compile the function in nopython mode.

[codecov[bot]]

Codecov Report

Merging #54 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   93.62%   93.62%   +<.01%     
==========================================
  Files          10       10              
  Lines        1223     1224       +1     
==========================================
+ Hits         1145     1146       +1     
  Misses         78       78

Impacted Files	Coverage Δ
mne_features/univariate.py	98.04% <100%> (ø)	:arrow_up:

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[agramfort]

thx @jbschiratti