Let users specify number of chunks in the config.

[codetalker7]

The indexer saves the index in chunks (see the index function for reference). However, the number of chunks into which the data is stored is figured out from the size of the collection and the chunksize from the get_chunksize function:

https://github.com/codetalker7/ColBERT.jl/blob/6e0f89f61abf421755f5201d39538d15f36d2321/src/indexing/collection_indexer.jl#L151

It will be good to let users control what chunksize they want to use; this will also be helpful for testing.

[codetalker7]

Update: was solved in #27.