SimpleVectorIndex.load() is easy to be called more than once

[FlyingDuck]

index/simpleVectorIndex

func (s *Index) IsEmpty() (bool, error) {
	...      
	err := s.load()
	...
}

func (s *Index) SimilaritySearch(ctx context.Context, query string, opts ...option.Option) (index.SearchResponses, error) {
	...
	err := s.load()
	...
}

The invocation of the load() function is very subtle, which easily leads to cases of repeated calls. Such as in the example.

We'd better make sure it's loaded only once.

[henomis]

Ok, added a task on the issue #116

[henomis]

Fixed in #123