Return dataframe of mapping sets, parameters etc. with ordered columns

[danielhuppmann]

Currently, calling Scenario.par() returns a pd.DataFrame with columns in no particular order.

This should be changed to using the order of Scenario.idx_names() + ['value', 'unit'] (for parameters) and ['lvl', 'mrg'] for variables and equations, respectively.

[khaeru]

Just realized this was done as part of setting up the Backend API / JDBCBackend in #182:

ixmp/ixmp/backend/jdbc.py

Lines 757 to 760 in 59e99e4

	# Retrieve the item
	item = self._get_item(s, type, name, load=True)
	idx_names = list(item.getIdxNames())
	idx_sets = list(item.getIdxSets())