Document support for in-memory HyperSQL databases

[zikolach]

It might be useful for testing and some other cases when having a persisted DB is not needed (e.g. following tutorials). It was originally added to #219 but due to lack of time for creating documentation/writing tests/etc was not merged.

[khaeru]

In #232 @zikolach added this file:

ixmp/tests/data/testdb/test.properties.sample

Lines 1 to 7 in 399e1b0

	config.name = unit_test_db@local
	jdbc.driver = org.hsqldb.jdbcDriver
	jdbc.url = jdbc:hsqldb:mem://ixmptest
	jdbc.user = ixmp
	jdbc.pwd = ixmp

…used in:

ixmp/tests/backend/test_jdbc.py

Lines 39 to 43 in 399e1b0

	def test_connect_message(caplog, test_data_path):
	sample_props = test_data_path / 'testdb' / 'test.properties.sample'
	ixmp.Platform(dbprops=sample_props)
	assert caplog.records[-1].message == \
	'launching ixmp.Platform connected to jdbc:hsqldb:mem://ixmptest'

This is a proof-of-concept & test that in-memory databases can be used. However, it is still not advertised, documented, or configurable (without use of a dbprops file).

[khaeru]

Note that as part of #270 we now make extensive (almost exclusive) use of this in the test suite, so it's very well-supported. The documentation/public API/command-line configuration still needs work.