Improve doc and warning message on numpy memory error

Improve documentation about dataset_adaptor.append(array, name) method. Specially, specify to cases of array:

• shape[0] is matching number of elements so array is used "as is"
• otherwise array is copied at each index of a newarray, where newarray.shape[0] is the number of element

In the second case, numpy may throw a MemoryException because of a too big memory requirement. Handle this to add explanation about the shape expectation.