Having problem with 'pandas.DataFrame.loc' for multi type and size dataframes.
The problem: 
When I make a data frame with pandas that has columns with different length and type, I cannot re-assign an element inside it using loc. I can explain it better with an example:

```python
import pandas as pd
import numpy as np

df = pd.DataFrame({'a': [np.array([1,2,3]), np.array([4,5,6]), np.array([7,8,9]), np.array([10,11,12]), np.array([13,14,15])], 'b':[5,5,12,123,5]})

df.loc[2,'a']= np.array([53,23,4])
```

The error that I receive is :
```python
Traceback (most recent call last):
  File "...", line 3326, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-67-4741bddaf261>", line 1, in <module>
    df.loc[0,'a']= np.array([53,23,4])
  File "...", line 205, in __setitem__
    self._setitem_with_indexer(indexer, value)
  File "...", line 547, in _setitem_with_indexer
    "Must have equal len keys and value "

ValueError: Must have equal len keys and value when setting with an iterable
```
Now If I remove the second column ('b') and make my data frame as
```python
df = pd.DataFrame({'a': [np.array([1,2,3]), np.array([4,5,6]), np.array([7,8,9]), np.array([10,11,12]), np.array([13,14,15])]})

df.loc[2,'a']= np.array([53,23,4])
```
I will not get any error. I also noticed that doing the assignment with

```python
df['a'][2]= np.array([53,23,4])
```
will do the work but throw a "SettingWithCopyWarning". However, I can go around the whole thing and use the following code to not get the error or the warning (thanks to ALS777 from [stackoverflow](https://stackoverflow.com/questions/60443110/why-in-pandas-dataframe-with-mix-elements-types-when-i-change-an-element-i-get)):
```python
df.at[2,'a'] = np.array([52,23,34])
```
#### Problem description

I am guessing that it should work with loc if it works with direct indexing like df['a'][2]. Also, why changing the other column effect the process of updating another column?

My specs are:

#### Output of ``pd.show_versions()``

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-40-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8
pandas           : 0.25.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 42.0.2.post20191203
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.2
sqlalchemy       : None
tables           : 3.6.1
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

