Array API: port straightforward functions in `observations`

[paddyroddy]

Is Your Feature Request Related to a Problem? Please Describe

• observations.gaussian_nz
• observations.smail_nz
• observations.fixed_zbins
• observations.equal_dens_zbins
• observations.tomo_nz_gausserr
• observations.deflect

Describe the Solution You'd Like

No response

Describe Alternatives You've Considered

No response

Additional Context

No response