Find a way of reading gnuplot output

When in a multicolor environment, throw an exception if the user draws more times than the number of subplots.
Also (I'm not sure this is a good idea) add the option to ignore this check.

Also when in multicolor environment other calls of multiplot shall not be allowed.

In addition to this: can subplots have child subplots? If so, the multiplot guard should have another subplot function or some similar mechanism

This is the current definition:

struct Gnuplotpp::Vector2d
{
    double x = 0;
    double y = 0;
};

it should be changed making the actual data private and exposing x() and y() methods so that is conformed to the LC library

instead of functions, allow users to create classes like ErrorBar : Plot2d che internamente reimplementa la scrittura su terminale

and add tests

TODO

Add the DataBuffer::reserve() methods to reserve space for a cestain number of rows.

Also add some suefult vector like functions for example

• crear
• capacity
• ...

So that it is easier to manage and is can easily be converted to shared_ptr,
For example:

auto p = Gnuplotpp::plot(/* ... */); // p is 'unique_ptr'
std::shared_ptr<Gnuplot::Plot2d> p2 = Gnuplotpp::plot(/* ... */); // also valid

Also take a look at custom manipuors

Add more terminals, for example windows, post script, latex, jpeg, svg, pdf, ... .
Use help terminal for info

We should introduce, fome mechanism to receive the gnulot output. For example:

• On whindows, a Named Pipe or a pseudo terminal PTY, PTY creation
• On Linux some sort of pipe duplication, fork, or PTY, also see gpReadMouseTest.c example
• No read when not available on the current system or streambuf, also MultiStream class should use the operator >> with only one of the streambufs and a function to get the output of >> for every streambuf that allows reading