dtype errors on using the model if keras backend is not set to float64

[hstojic]

Default dtype for keras is float32 while gpflow uses float64, so if one tries to use the model without setting the keras backend to float64 dtype clash will occur and exception is raised,

The problem with setting the backend occurs when multiple models with both float32 and 64 are used in a library, as its a global setting. The fix seems to be easy, fixing lines 99 and 100 by setting the dtype to tf.float64.