Fix errors in __init__ and set_data, add calc_aggregate method
Fix errors in __init__ and set_data, add calc_aggregate method

- Fix error when passing str as grid_element in __init__
- Fix pb in set_data (created an array when changing a str value)
- new method calc_aggregate (first version, to be modified)

amandersillinois/landlab,cmshobe/landlab,cmshobe/landlab,landlab/landlab,amandersillinois/landlab,cmshobe/landlab,landlab/landlab,landlab/landlab