Make user-facing code use scico.random instead of np.random

[Michael-T-McCann]

Address the part of https://github.com/scici/scico/issues/315 that I think is worth fixing now. There remain uses of np.random in the tests.

[codecov]

Codecov Report

Merging #235 (3953842) into main (244b05c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #235   +/-   ##
=======================================
  Coverage   92.52%   92.52%           
=======================================
  Files          49       49           
  Lines        3452     3452           
=======================================
  Hits         3194     3194           
  Misses        258      258           

Flag	Coverage Δ
unittests	92.52% <ø> (ø)

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Impacted Files	Coverage Δ
scico/blockarray.py	91.20% <ø> (ø)

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