Document & extend interface for ideals, add conformance tests

[fingolfin]

Motivated by comments in PR #1731

Should be in docs/src/ideal_interface.md I guess?

[fingolfin]

Some requested APIs from #2038 (some of these are already there in Hecke)

• is_principal
• is_prime
• is_primary
• is_maximal

[fingolfin]

Maybe @emikelsons can look into this once he is finished with his dim work.