Replace assert to check int in get_el_sp

[DanielYang59]

Currently get_el_sp uses assert to check if obj is integar, this could be bypassed by the optimization flag (-O) and results in surprising behaviour, for example:

python3 -c "from pymatgen.core.periodic_table import get_el_sp; print(get_el_sp(4.5))"
>>> ValueError: Can't parse Element or Species from 4.5

However with -O it bypasses assert and gives Be because of int(flt):

python3 -O -c "from pymatgen.core.periodic_table import get_el_sp; print(get_el_sp(4.5))"
>>> Be