Single particle model

Framework implemented for uniform (single particle) soil.

Lacks setup.py and full test coverage.

----------- coverage: platform win32, python 3.8.5-final-0 -----------
Name                         Stmts   Miss  Cover
------------------------------------------------
DHLLDV\DHLLDV_Utils.py          22      0   100%
DHLLDV\DHLLDV_constants.py      11      0   100%
DHLLDV\DHLLDV_framework.py     226     91    60%
DHLLDV\__init__.py               0      0   100%
DHLLDV\heterogeneous.py         55      1    98%
DHLLDV\homogeneous.py           55      3    95%
DHLLDV\stratified.py            77      9    88%
------------------------------------------------
TOTAL                          446    104    77%