Improve API for Julia 1.0.2 using keyword arguments

[diegozea]

• deprecate @residue... macros and use kw args instead
• make map...freq simpler with kw args and default values

[diegozea]

MIToS v2.20.0 has deprecated the @residues macro and the residues function in favor of select_residues that use keyword arguments.

[diegozea]

MIToS v.2.22.0 makes easy the API of the functions defined in the Information module by using keyword arguments and sensible defaults. In particular, it now defines the mapfreq function and uses it to define information measures, such as shannon_entropy, mutual_information, on multiple sequence alignments.