This function combines the functions read_misa_oneSite and read_misa_multipleSites and is strictly bound the the misa folder structure
read_misa_files(input_path)This is the directory where the two folders "files_per_site" and "sites_per_file" are located
A Data frame with 3 columen: Timestamp, Oxygen data and Site name
All csv files from both folders will be loaded. They must contain a timestamp column and an oxygen column. The timestamp column is identified automatically, by looking for a column where the entries contain ":" and one of the date separating symbols ".", "/" or "-".. The oxygen column is found by its colname. It should conatin "O2", "o2", "Oxygen", "oxygen", "ox", "Ox", "Sauerstoff" or "sauerstoff".
For "file_per_site" files: all letters in the filename before the first "_" are used for the sitename. For "sites_per_file" files: all column names will be used for site names. Thus, all columns except the timestamp must be oxygan concentrations at different sites