Simplify Isotope repr

[tjlaboss]

Let's say we have an O-16 Isotope in the 01c xslib. Currently,

>>> str(o16)
' O-16  (01c)'
>>> repr(o16)
'ZAID=8016, Z=8, A=16, element=oxygen, library=01c'

__repr__ is mostly used for debugging and logging purposes and should remain simple.

Describe the solution you'd like

Move the current __repr__ to __str__, and do the following instead:

>>> repr(o16)
'Isotope(8016.01c)'

Describe alternatives you've considered
I think the Isotope({self.ZAID}) is best, because ZAID is the most widely used convention in MCNP [citation needed]. However, the new MCNP6.3.1 style would be acceptable as well:

>>> repr(o16)
'Isotope(O-16.01c)'

Single or double quotes around the identifier may be preferable as well: Isotope(". . .") or Isotope('. . .')

Additional context
More broadly, the __repr__ functions should be simplified throughout MontePy to be brief and to show the class name. I'm targeting this one first because of #472.