Standardized MOAs and More!

[drbergman]

Many changes here:

• standardized moas
• two drugs diffuse in from ymin
• moved PD parameters to cell definitions
• organized remaining user parameters
• coloring based on damage from both drugs
• two tumor types included in mymodel.xml
• oxygen included as substrate
  • this can be manually removed in custom.cpp or by setting pO2 threshold parameters so that cells do not respond to low oxygen levels
  • oxygen diffuses in from all sides
  • lack of oxygen causes lower proliferation and necrosis
• cells now start in a disc rather than across the whole square
• and possibly more