[FR] Add 'positive_only' flag to coefficients

**Is your feature request related to a problem? Please describe.**
Often only the positive part of the spectrum is required (because its symmetric anyway)

**Describe the solution you'd like**
Add a flag `postive_only` to `sample_coefficients` method in `Coefficients` class which causes the returned value only containing the right halve of the array including the zero coefficient.

**Describe alternatives you've considered**
N/A

**Additional context**
N/A

Locate the Coefficients class and its sample_coefficients method, then inspect how the coefficient array is assembled and how its zero coefficient is represented. Implement the requested positive-only option so the returned array contains the right half including zero, and verify the existing behavior remains unchanged when the flag is not used.

Written by the indexing model from the issue text.