Aliens — Refugees — Definition — Fear of persecution — Political opinions — Right to asylum on grounds of political persecution — Requirement of causal link between political persecution and flight — Extent to which post-flight circumstances may be taken into account — Distinction between objective and subjective post-flight circumstances — “Self-created” post-flight circumstances — Requirement that political opinions reflect firm conviction clearly manifested prior to departure from country of origin — The law of the Federal Republic of Germany