New methods for estimating the embedding dimension (#9)

New methods for estimating the embedding dimension (#9)

• faster _average_a

• new fnn-based methods for estimating dimension

• fix REQUIRE

• same return types of afnn, fnn and f1nn

• interface and doc of estimate_dimension

• fix in signature of estimate_dimensions

• test estimate_delay methods

• add citations, (Atol, Rtol) -> (atol, rtol)

Julia functions that use tolerances use atol instead of Atol. In general, CamelCase is reserved for type and module names while it is preferred that parameter and keyword names use lower case.