Use less transactions

[Christian-B]

This Pr is based on ideas from #1062

The purpose is to reduce the number of database transaction.

The basic idea is that all database patterns follow the pattern
with xyzDatabase() as db:

• The enter starts a transaction and caches a cursor
• SQLiteDB has a few pass through methods which check the cursor/transaction exists and uses that
• Exit commits the transaction and nukes the cached cursor so it can not be reused.

Most changes are just tabbing after the removal of the
with self.transaction() as cursor:

While NOT recommended it is possible to have two xyzDatabase Objects open on the same file
BUT if you write to both it goes BOOM!
see test_double_with

The pattern
with DsSqlliteDatabase() as db:
dsg = DataSpecificationGenerator(0, 1, 3, vertex, db)
looks strange in tests but was used so that
_GraphDataSpecificationWriter runs the whole generation in a single transaction

A few code changes includes:

• The View now holds the path of the DsSqlliteDatabase
• Database queries moved from SpinnMan to Fec

Must be Done at the same time as:
SpiNNakerManchester/SpiNNMan#361
SpiNNakerManchester/sPyNNaker#1385

[coveralls]

coverage: 42.293% (-0.2%) from 42.508% when pulling b02f10e on with_database into 9a83667 on master.