location of STLSpecificationParser

[TomyYamy]

I think current location
rtamt/rtamt/ast/parser/stl/discrete_time/specification_parser.py
in
commit: c2b6dbd branch ast-refactoring
is pretty good. But let's keep in mind.

[TomyYamy]

I would like to change the structure a little bit.
Please see, my commit: 5f7d5a8 branch STLSpecificationParser.
I find we have
rtamt/rtamt/ast/parser/stl/discrete_time/specification_parser.py
rtamt/rtamt/ast/parser/stl/dense_time/specification_parser.py
and main implantation done in discrete_time and dense_time overwrite only visitConstantTimeLiteral().

regarding #6
2. extend a few operator to existed TL (like STL).
some library use want to implement only dense_time. The code gives misunderstanding we need to implement discrete_time in any case. However, in reality, discrete_time or dense_time only effect visitConstantTimeLiteral() and others are common.
In that sense, my suggestion is,
rtamt/rtamt/ast/parser/stl/specification_parser.py : put almost of all method
rtamt/rtamt/ast/parser/stl/discrete_time/specification_parser.py : discrete_time version of visitConstantTimeLiteral()
rtamt/rtamt/ast/parser/stl/dense_time/specification_parser.py : dense_time version visitConstantTimeLiteral()

I did not give pull-request. I did not check it works or not because of luck of issue #89

[TomyYamy]

I merged the branch.