mywrangle.py: fixed iso dataframe bug

In read_iso_darth, the iso pandas dataframe is
cut to only include masses within the minimum
and maximum masses specified in the arg call.

pandas dF's do not get reindexed when cut, so
there were problems when looping over the elements
of any series of the dataframe, i.e. when calling
element '0', such element did not exist, because
it had been eliminated by the mass cuts.

Simple fix: After mass cuts, reindex dF using
reset_index() method.

reset_index() is crucial for any cuts made on
pandas dataframes if looping over the indices,
will check this throughout all the code in a
future commit.