Switch hemisphere constants to be in line with AIBS

[mgeplf]

• Based on this: https://github.com/AllenInstitute/CCF_Tutorial/blob/63914f6c4b666428d3e2f2bd9b934519d23581dd/CCF_Tutorial.ipynb "We'll use a hemisphere id of 1 for the left hemisphere and 2 for the right."

The hierarchy file also contains hemisphere ids, but only '3' is used, likely to specify 'both'.