Hotfix/v0.2.4

[AmroAlJundi]

A few small fixes to address the usage report done by @konstapo.

• Commented out prints in the sparsebase::feature::Extractor and sparsebase::object::Graph classes (until verbosity is formally added to the library).
• Made the "weighted" parameter in Matrix Market readers non-optional. This will force users to explicitly state whether the matrix market file they are reading contains weights or not. This is a temporary solution until a reader that infers this information automatically is added (effort already started in Code Suggestion for CSR to MTX File Input / Output #125).
• Added functions to clear conversion functions from sparsebase::utils::converter::Converter classes, as well as some tests related to them.