Accessor naming convention

While I thought we did currently we don't specify a specific format for accessors. An exception for data member accessors is useful, it is useful for both searching the code and ease of reading.

I propose when a method is a simple accessor i.e. a get or set that we avoid
```
VariableType getVariableName() const { return variable_name_; }
```
in favor of
```
VariableType get_variable_name() const { return variable_name_; }
```

In the case instances where the accessor results in a calculation and the member variable serves as a cache of the answer the normal convention holds.
```
VariableType getVariableName { if ( variable_name_ is dirty )
{ ... }
return variable_name_; }
```