bioset 0.2.0

• add sets_read
• add option to import csv with "," as decimal point
• calc_conc / calc_prefix / convert_conc / convert_prefix:
  • add more rigorous checks on input
  • will throw an error if the unit is not recognised
  • be more permissive with whitespace in input
  • add more tests
  • permit more metric prefixes
  • add more examples
• fix error in factor_conc:
  • factor_conc did not stop if molar_mass / density_solute /
    densitiy_solution were required but missing
  • add unit tests