Create series_7400_tests.py

Added tests for the 50 ICs of 7400 series .
Corrected a bug for IC 7445 and a few typos.
added file name of series7400 testfile..
Wrong usage of 'cos' instead of 'c'
Added a Exception and two more cases

For Ohmslaw_AC() :
- the case when 'p' and 'c' are unknowns is a exception.
- the cases when 'c,z' and 'c,i' are unknowns wernt considered.
Merge pull request #139 from mohitgujarathi14/develop

Added test file for series7400ICs and corrected a bug