add a public generate_parameters function

[anilbey]

EModelRunner and e-model-packages have been depending on the private _generate_parameters.

It's recent signature change cause failures in the dependent packages.

This PR adds a public function so that the dependent packages won't have to worry about bluepyopt internals.

https://github.com/BlueBrain/EModelRunner/blob/ff5607b9edbe4db1aefe2888ac5fa4fd1d664e2e/emodelrunner/create_hoc_tools.py#L552

[codecov-commenter]

Codecov Report

Merging #442 (52ea4e1) into master (af06928) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #442      +/-   ##
==========================================
+ Coverage   87.90%   87.91%   +0.01%     
==========================================
  Files          50       50              
  Lines        4281     4285       +4     
==========================================
+ Hits         3763     3767       +4     
  Misses        518      518              

Impacted Files	Coverage Δ
bluepyopt/ephys/create_hoc.py	92.53% <100.00%> (+0.22%)	:arrow_up:

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