Updating full matlab model (2D only)

This commit has full working version of the model in 2D. The bulk of
the model is located in the “mainfunctions” dir as two scripts
(ActinMultiplex.m and MAINBOX.m) which can be launched using their
respective GUI files.

Model components and workflow:

1. ActinMultiplex.m
- renders actin filament networks according to empirical work on actin
dynamics in dendritic spines
- real-time visualization of filament network formation
- misc statistical and graphical outputs
- packages tip locations into cell matrices for export

2. MAINBOX.m
- filament locations get imported by mainbox and are used in
simulations of scaffold protein (Sap) dynamics at synapses
- simulates surface receptor diffusion in-and-around dendritic spines,
and between spines
- simulates surface receptor interactions with submembrane Sap
particles, which transiently affect diffusion rates, leading to
synaptic accumulation of surface receptors
- analysis of steady state dynamics, curve fitting, density analysis
- misc statistical and graphical outputs