Add support for JUNG and JGraphT to Java library

This commit adds the capability to covert FNSS Topology objects into
JUNG and JGraphT graph objects. JUNG and JGraphT are the most used
network graph libraries for Java. Converting FNSS topologies into
JUNG or JGraphT objects enables to use their implementations
of graph algorithms, such as Dijkstra algorithms and many more.

This commit adds the source of the classes implementing this
functionality as well as examples, test code and Jar files of the
new dependencies.

As a result of the implementation of this new feature, the FNSS Java
library depends on more third-party Java libraries: JUNG and JGraphT
as well as their dependencies.
Move unit of measures conversion to dedicated module

This commit creates a new module in the core component to hold
units conversion functions and dictionaries. This commit moves
this code from the util module.
Minor refactoring regarding import of release module

This commit changes the way the release module is imported fnss
is imported. This change reduces variables pollution by preventing
the import of all members of release into the fnss module namespace.
Minor refactoring and typo corrections around the code
Rename servers to hosts in datacenter topologies

This commit renames all references to 'servers' into references
to 'hosts' in datacenter topology code. This change has been carried
out to align the naming to Mininet naming for ease its future
integration.
Remove all star imports from test classes

This commit remove star imports (not recommended) by the test
modules which were using them.
Replace calls to type with isinstance

This commit replaces in all Python code calls to the 'type' function
with calls to 'isinstance' when used in comparison. For example,
'if type(x) is int' becomes 'if isinstance(x, int)' with this commit.
This changes has been made because the use of type in such situations
is deprecated.