Improve consistency across command lines

[thibaultdvx]

Many command lines use <...> format for the arguments, e.g. clinicadl tsvtools analysis <merged_tsv> <data_tsv> <results_path>, whereas others use capital letters, e.g. clinica run pet-linear [OPTIONS] BIDS_DIRECTORY CAPS_DIRECTORY ACQ_LABEL {pons|cerebellumPons|pons2|cerebellumPons2}.

Let's use capital letters to be consistent with the documentation of Clinica and ClinicaDL.