Update Isotope.half_life_unit values

[lmmentel]

Isotope half life units are updated to be unambiguous, following these substitutions:

unit_subs = {
    'Ey': "Eyear",
    'Gy': "Gyear",
    'My': "Myear",
    'Py': "Pyear",
    'Ty': "Tyear",
    'Yy': "Yyear",
    'Zy': "Zyear",
    'as': "asec",
    'd': "day",
    'h': "hour",
    'ky': "kyear",
    'm': "minute",
    'ms': "msec",
    'ns': "nsec",
    'ps': "psec",
    's': "sec",
    'us': "usec",
    'y': "year",
    'ys': "ysec",
    'zs': "zsec",
}

With this change units can now be correctly parsed by pint.

Changes

• updated values in isotopes.half_life_unit according to substitution rules above
• added a test to check that all values are substituted
• added annotation in propertymetdata.annotations for row with isotopes.half_life_unit with information from the source paper that " 1 year = 365.2422 days = 31 556 926 sec"

fixes #157