AeroDist doesn't throw an exception for "sampled" type modes when other modes are present

[Griger5]

Code example:

import PyPartMC as ppmc
from PyPartMC import si

from unittest import TestCase

aero_data = ppmc.AeroData((
    {"OC": [1000 *si.kg/si.m**3, 0, 1e-3 *si.kg/si.mol, 0.001]},
))

sampled = {
    "mass_frac": [{"OC": [1]}],
    "diam_type": "geometric",
    "mode_type": "sampled",
    "size_dist": [
        {"diam": [1, 2, 3, 4]},
        {"num_conc": [100000, 200000, 300000]},
        {"not_used": [0]}
    ]
}

other = {
    "mass_frac": [{"OC": [1]}],
    "diam_type": "geometric",
    "mode_type": "log_normal",
    "geom_mean_diam": 8.64 * si.nm,
    "log10_geom_std_dev": 0.28,
    "num_conc": 2900 / si.cm**3
}

# D1 throws an exception, as it should
with TestCase.assertRaises(TestCase, Exception):
    D1 = ppmc.AeroDist(aero_data, [{"mode1":sampled}])

# this assertion should pass with the issue resolved
with TestCase.assertRaises(TestCase, Exception):
    D2 = ppmc.AeroDist(aero_data, [{"mode1":other, "mode2":sampled}])

When the size_dist parameter of a sampled-type mode has more than 2 dicts (or the dict keys aren't "diam" and "num_conc") it should throw an error. This works when the sampled mode is the first mode in AeroDist, however, it fails to throw an exception when modes of different types are present before it.

The code above should pass without errors after the issue is resolved.