Investigate making package work with table with no Primary Key

[rotexdegba]

• Make API usable with tables that do not have a primary key column defined.

  Remove the phrase Working on supporting tables that do not have any primary key column defined from README.md once the above task is completed

  Remove a similar variant of the earlier mentioned phrase from the doc-block of \GDAO\Model->_primary_col

  Change the default value of \GDAO\Model->_primary_col to null in its declaration

  Also change the phrase **This is a REQUIRED field & must be properly set by

  consumers of this class** in the doc-block of \GDAO\Model->_primary_col to

  • This is an OPTIONAL field & must be properly set by consumers of this class