Minor stylistic and code changes for consistency

The more significant changes are:
* Removed redundant return statements
* Moved and reformatted MP6 methods for consistency with wb.h
Removed underscore prefixes from wbImage_t struct members

Rationale: Variables with a leading underscore are reserved as global identifiers by some implementations.

Renamed wbImage_t struct members to eliminate redundancy in expressions. Compare

int dataSize = image._imageWidth * image._imageHeight * image._imageChannels;

to

int dataSize = image.width * image.height * image.channels;
Placed parseCsv() and wbImage_save() into namespace wbInternal {}; renamed parseCsv() to wbParseCSV()
Reflowed wbParseCSV() so that the file stream status check immediately follows its initialisation