NatLibFi/Annif: Annif 0.50

This release introduces a setting to use only a part of the input text for subject indexing: the new input_limit project parameter truncates the input text to the given character number. This can improve the quality of the suggestions as the beginning of a long document typically includes an abstract and introduction. The default value for input_limit is zero, which means that truncation is not performed. Improvements include better handling of cached data in nn_ensemble training and optimization of memory usage in evaluation by using sparse matrices for suggested subjects. Many dependencies have been updated and a few minor issues fixed. New features: 446 Add a backend paratemer to limit input characters in suggest 452 Apply the input_limit backend parameter to texts in train & learn Improvements: 441 Sparse subjects (credit @mo-fu) 443/#444 Allow use of cached data after cancelled training of nn_ensemble backend Maintenance: 448 Upgrade dependencies 445 Upgrade LMDB dependency from 0.98 to 1.0.0 449 Resolve DeprecationWarning: change warn to warning Bug fixes: 447 Fix missing default params in pav and nn ensembl