MNT: Remove deprecated np.float usage

[effigies]

Wanted to run this one by people rather than tucking into #3336.

Because np.float is just __builtins__.float, which is in turn promoted to np.float64 when used as a dtype, this should have no effect. Just want to make sure that this is what's intended in these cases.

[codecov]

Codecov Report

Merging #3341 (cbc1824) into master (d02750a) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master    #3341   +/-   ##
=======================================
  Coverage   65.01%   65.01%           
=======================================
  Files         302      302           
  Lines       40035    40035           
  Branches     5291     5291           
=======================================
  Hits        26029    26029           
  Misses      12935    12935           
  Partials     1071     1071           

Flag	Coverage Δ
unittests	64.79% <50.00%> (ø)

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Impacted Files	Coverage Δ
nipype/algorithms/rapidart.py	34.42% <0.00%> (ø)
nipype/algorithms/modelgen.py	59.25% <100.00%> (ø)

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