fixed problem with Hom

A test: test/tensor.m2 now fails, but this is because it is doing
bad things wrt chain complexes.
Started to reorganize mutable matrix code.  Did NOT check tests here.
Merge branch 'master' of github.com:Macaulay2/M2
added 'isCanceled'-method  for a task
	modified:   Macaulay2/d/actors4.d
	modified:   Macaulay2/m2/exports.m2
Merge pull request #8 from jakobkroeker/master

added check if a task was cancelled
Merge branch 'master' of github.com:Macaulay2/M2