Rename mad() in mean_absolute_deviation()
#### Problem description

MAD is a overloaded shortcut: It stands for *M*edian *A*bsolute *D*eviation (more common), but also can be used as *M*ean *A*bsolute *D*eviation (which is used in pandas). 

#### Expected Output

To avoid confusion, the method should be renamed from `.mad()` to `.mean_absolute_deviation()`, and when implemented the more commonly used `.median_absolute_deviation()`.

If you are fine with it, I would kick off a PR. 

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

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.3.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-24-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 0.25.0
numpy            : 1.17.0
pytz             : 2019.2
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 41.0.1
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.0
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.7.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.0
matplotlib       : 3.1.1
numexpr          : 2.6.9
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : 1.3.6
tables           : 3.5.2
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
```
</details>
