Allow API functions to be used without loading hoc/mod #minor

[anilbey]

Previously loading the hoc and mod files were taking place during import bluecellulab.

This was coupling every function in the API with the hoc and mods. With this change only the creation of SSim, Cell and RNGSettings objects is triggering the loading of mod and hoc files (once and only once).

Minis-validation tests pass:

  py310: OK (125.48=setup[66.64]+cmd[11.43,47.41] seconds)
  congratulations :) (125.69 seconds)

PSP-validation tests pass:

74 passed, 87 warnings in 334.98s (0:05:34) 
  py310: OK (408.65=setup[68.46]+cmd[4.54,335.65] seconds)
  congratulations :) (408.86 seconds)

[ilkilic]

Looks perfect!