refactor: remove seaborn as a dependency

[danielolsen]

Purpose

Avoid requiring us to load plotting libraries (seaborn and matplotlib) when all we really need is a string that specifies a color for matplotlib in PostREISE.
Closes #87.

What is the code doing

Specifying our xkcd colors in an equivalent way, without needing to import seaborn to do it.

Removing seaborn from our requirements list.

Time to review

Extremely quick, it's less than 15 lines of changes and most of them are doing the same thing.

[rouille]

Also, after rebasing onto develop, I created a new environment and did pip install -r requirements. The pip list command shows that seaborn is not installed:

(PowerSimData) [~/REM/PowerSimData] (remove_seaborn) brdo$ pip list
Package            Version
------------------ -------
asn1crypto         1.3.0  
atomicwrites       1.3.0  
attrs              19.3.0 
bcrypt             3.1.7  
cffi               1.14.0 
cryptography       2.4.2  
geographiclib      1.50   
geopy              1.20.0 
idna               2.9    
importlib-metadata 1.6.0  
jsonpickle         1.2    
more-itertools     8.2.0  
numpy              1.16.0 
pandas             0.25.3 
paramiko           2.4.2  
pip                20.0.2 
pluggy             0.13.1 
py                 1.8.1  
pyasn1             0.4.8  
pycparser          2.20   
PyNaCl             1.3.0  
pytest             4.5.0  
python-dateutil    2.8.1  
pytz               2019.3 
scipy              1.2.0  
setuptools         39.0.1 
six                1.14.0 
tqdm               4.29.1 
wcwidth            0.1.9  
zipp               3.1.0  

and, finally, ran the tests. They all pass.

[rouille]

Looks good. Thanks for the improvement. Please rebase onto develop before merging.