Update filewriter.h

Added some new fuctions:
0. size_bytes gives you the information about the size of a file in bytes
1.openall opens a file and reads out all values into an array
2.openfrac opens a file and reads out a fraction of it. You have to specify the length of the 
 fraction and the index (e.g fraction = 100 ; index = 2   will read the data from 100 to 200.)
3. splitarray can be used to split an array into smaller arrays.