mounting an sector reading implemantations.

+ implement Disk::mountdisk(string &)
+ implement Disk::unmountdisk( )
+ implement Disk::readSector(int, Sector*)
+ implement Disk::readSector(Sector*)
+ added new function to Disk, called savechanges() used in
Disk::mountdisk(string&)
* eddited Disk::createdisk(string&,string&) to implement savechanges()
* a few errors editted
* some other small edits.