#16 VNUML checking for concurrent simulations

[Fermín Galán]

VNUML allows to run several concurrent simulations,
each one described in a separated VNUML source file.
However, to avoid interference between simulations, the
following rules have to be followed:

* Each simulation must have a diferent name
(<simulation_name>).
* Virtual machines and network names (name
attribute of <vm> and <net>) must be unique for all
simulations. The same name in diferent simulations is
not allowed.
* Management IP addresses for the management
interface must not overlap in two or more simulations.
To avoid this, use <ip_offset> with a different and
suitable value in each simulation (it depends of the
number of UMLs in each simulation). For example
<ip_offset>0</ip_offset> in the one simulation,
<ip_offset>10</ip_offset> in another.
* To interconnect machines of different simulations
to the same network, the same network name have to be
defined in both simulations. Thas it, the <net> of both
simulations for the desired networks must be the same.
* Avoid using duplicated MAC addresses. If using
<automac> uses a diferent offset in each simulation.
* Due to the fact that all simulations share the
host enviroment (<host> tag), some conflicts could
arise. The recomendation is not to use <host> tag at
all whenever possible.

It would be desirable that the parser could perform
those checking, so the user doesn't need matter about them.