bug fix for 01a when "use_all_zones = True"

[pjuckem]

Currently, notebook 01a will throw an error if "use_all_zones" is set to True. The proposed edits below should allow it to work properly:

Code block 15 ("#create grid cell dimension arrays"):
....
for group in zone_df:
if group == 'all_zones:
sat_vol = sat_vol_cell.sum()
recharge = flow_sum[0].sum()
else:
sat_vol = sat_vol_cell.ravel()[zone_df[group]].sum()
recharge = flow_sum[0, zone_df[group]].sum()
....

Code block 16 (Generate random particle placement):
....
for group in zone_df:
if weight_scheme == 'flow':
if group == 'all_zones:
weight = flow_sum[0]
else:
weight = flow_sum[0, zone_df[group]]
weight_label = 'flux'
elif weight_scheme == 'volume':
if group == 'all_zones':
weight = sat_vol_cell.ravel()
else:
weight = sat_vol_cell.ravel()[zone_df[group]]
weight_label = 'volume'

f = number_of_particles_per_group / weight.sum()
parts_per_cell = np.rint( weight * f ).astype( np.int32 )

l, r, c = np.indices(( nlay, nrow, ncol ))
if group == 'all_zones':
    l= l.ravel()
    r= r.ravel()
    c= c.ravel()
    label = zones
else:
    l= l.ravel()[zone_df[group]]
    r= r.ravel()[zone_df[group]]
    c= c.ravel()[zone_df[group]]
    label = zones[zone_df[group]]

....