Internal usage of char arrays only or prepare to use chars and strings

Since Matlab can produce a lot of problems when a char array is expected and a string is used (string array vs char array in concatenation, and different results in some functions) Three promising strategies emerge:

• catch all inputs or text and convert it to char (char()) as early as possible
• enable all functions to handle both chars as well as strings at each spot
• for all "usage" of text use functions that handle both and output strings, or convert to strings when using

Related to #81 (closed).