Statistics for “Na mata ni Civa au a vakawaletaka” : an ethnobotanical study on kumala (Ipomoea batatas (L.) Lam) and its contribution to climate-smart agriculture in Ra, Fiji : a thesis presented in fulfilment of the requirements for the degree of Doctor of Philosophy in Horticulture, Massey University, School of Agriculture and Environment, Palmerston North, Aotearoa