Rewrite MATLAB import functions with readstruct()

[Jerry1144]

One path to #343, rewriting MATLAB import functions with readstruct(). Shifting to MATLAB's own capabilities helps alleviate Java heap size problems. Force push of #344

Specific to this path:

• readstruct reads everything, so I let trackmateFeatureDeclarations() be the only place (among Spots, Edges, and Graph) that reads the XML, and pass that around as a secret global variable.
• struct field names may have been changed (according to MATLAB identifiers rule). There's additional logic to guess the original XML Attribute name. May consider moving to a separate function for code reuse.
• readstruct attempts to detect data types of attributes, so there are tests for "is it imported as double" littered everywhere.
• Updated to use strings throughout. Output formats remain unchanged.

• trackmateSpots.m
• trackmateFeatureDeclarations.m
• trackmateEdges.m
• trackmateGraph.m
• trackmateImageCalibration.m won't do it's silly to read everything under the <Model> node, just to fetch its attributes, but there's no choice of "read attribute only" from readstruct(). I see no reasonable way to do it. not matlab.io.xml.dom.* either, which isn't there yet.
• importTrackMateTracks.m