Refactoring: small fixes and improvements

Closes #71

See merge request !79

Also removed unnecessary getVectorLength function from getEntityMesure.h

Fixes #71

- the sum function was unused
- inline is useless for template functions

CUDA 11.2

See merge request !78

The condition is always satisfied, since getAllocatedElementsCount
returns the size of the values vector view, which was bound just prior
the assert.

Documentation: reorganize README.md and the documentation front page

See merge request !77

It is superseded by the usage hints documented on the front page and by
the linked example project.

- README is shorter, includes only an introduction and links to the full
  documentation for details
- New documented topics: optional dependencies, compiler flags,
  environment variables