Put constraints in a dictionary

This matches more closely the need to access constraints by axis, e.g.
for plot_ax, update_ax, but also for using the generalized matching
rules.

Resolves #55
Use DrawConstraints style from config file

Resolves #51
Define element selection rules in config file

Resolves #61
Perform one-point matching by default

Resolves #50
Allow several element parameters to be varied

Resolves #57
Merge branch 'matching-improvements'