Increase coverage and some minor fixes.

[Christian-B]

Code changes:

AbstractSingleSourcePartition.add_edge do check before changing state

ApplicationGraph
add_edge: do check before changing state:
add_outgoing_edge_partition: now private and all safety checks removed
get_edges_starting_at_vertex: removed as unused

ApplicationFPGAVertex.incoming_fpga_connection now yields nothing rather than raise a weird exception if no connections

ConstrainedObject removed check for None as isinstance fails on None

ConstantSDRAMMachinePartition.get_sdram_size_of_region_for
simplified to use known value

DestinationSegmentedSDRAMMachinePartition.
raise exception rather than return None for don't know

SourceSegmentedSDRAMMachinePartition.
do check before changing state
add_edge
@sdram_base_address.setter

SplitterExternalDevice.set_governed_app_vertex
Removed if as always True on a generator

SplitterOneToOneLegacy.set_governed_app_vertex
do check before changing state

MultiRegionSDRAM.nest bug fix for other types

ReverseIPtagResource eq: bug fix for other types

merged_routing_table_generator
- next as a property fails if using a debugger as debugger removes elements.
replaced next with pop method
made peek a method for consitency

• removed unused create_entry method

RoutingTree
removed a setter
fixed type in docs

Added lots of unittests

Some of the code not tested will be remove in or with
#460
#465
#464

tested by:
SpiNNakerManchester/IntegrationTests#141
(note other prs tested there can be merged individually)

[dkfellows]

Couple of questions. Other stuff looks fine.