docs: update README.md
docs: update vle.reader markdown file
docs: add vle.tester and update docs
docs: add doc from previous tex files

- vle.discrete-time
- vle.extension.differential-equation
docs: fix vle.discret-time figure
vle.reader and vle.tester: add a test package

- update of TableFileReader API
- add a new test package vle.tester_test
- fix vle.recursive_test
vle.reader: add the undo readLine function

- undo readLine function allow to reset the position of the stream at the
  beginning of the line.
vle.discrete-time.generic: add a model

- GenericMean performs dynamicaly the mean of inputs.
vle.discrete-time: add boolean for sync

- the sync condition can now be parameterized with a boolean
FALSE is equivalent to 0, and TRUE is equivalent to 1.
vle.discrete-time: add tests for access to Vect.

exception are sent when there is a wrong access to a Vect.
vle.discrete-time: allow operator for init

Var::operator= and Var::operator() respectively sets and  gets the initial
value in the constructor.
vle.reader: compute the status code exit

- the status code exit when parsing was not computed
- some useless data files have been removed in the test package
vle.discrete_time: fix history_size

history_size was taken as a number of unit time whereas it should be take as a
number of members into the queue.
vle.tester_test: fix bad path for the test file