Renamed variables in wb.h for consistency

Though the changes are mostly pedantic, they do help to clarify some of the following situations:

* The distinction between 'inputFile' and 'fileInput' in wbImport() and 'outputfile' and 'outputFile' in wbImage_save()
* In wbImage_t the variable 'data' is of type float and represents the data that is to be (or has been) convolved; in wbImport() the variable 'data' is of type unsigned char and represents the data in its 'raw' state
* The variables 'data' in wbImport() and 'rgbData' in wbImage_save() both refer to data in its 'raw' state so they have been renamed to 'rawData'
Removed redundant comments
Replaced literals in the OS X and UNIX/POSIX timing routines with their respective constants
Refactored wbImport() for MP1, MP4, & MP5 to improve error handling and reporting
Refactored wbImport() for MP2, MP3, & MP6 to improve error handling and reporting
Removed unnecessary cast of pointer returned by wbImport() in wbSolution()

wbImport() for MPs 1-5 has return type (float*)
Refactored wbParseCSV to improve error handling and reporting

It is not possible to implement error checking using the function atof() since it returns 0.0 on encountering an error; refactored to use streams
Refactored wbParseCSV to return a pointer memory created by malloc()

The Coursera MP templates call free() on the pointer returned by wbParseCSV(); it is undefined behaviour mix new with free() or malloc() with delete
Prevented zero-length memory allocation in wbImage_t constructor
Reflowed wbImport() so that the file stream status check immediately follows initialisation
Replaced the 'magic' numbers and multiple definitions of image 'channels' with the constant kImageChannels
Replaced the 'magic' number '255' with the constant kImageMaxval
Annotated assertions with error messages
Simplified the reading of image headers and comments in wbImport()
Simplified the reading of image dimensions in wbImport() to allow for error checking

It is not possible to implement error checking using the function atoi() since it returns 0 on encountering an error and this value can't be differentiated from parsing and returning the value 0; rewritten to use streams
Implemented basic error checking in wbImport() for image files
Refactored wbImage_save() to use in-core formatting

* Renamed the saved image to 'convolved_image.ppm'; Regardless of whether the image is 'convoluted', it has been 'convolved' by the MP
* Removed the warning regarding "Conversion from string literal to 'char *'" by declaring 'fName' const
Minor stylistic and code changes for consistency

* Qualified parameters that should be invariant with 'const'
* Use prefix increment exclusively
* Pass structs that are parameters as references
Included file name in wbSolution() error messages
Fixed memory leak in wbSolution()