ImmutableMatrix improvements

[isuruf]

  File "symengine/lib/symengine_wrapper.pyx", line 2133, in symengine.lib.symengine_wrapper.DenseMatrixBase.applyfunc (/home/isuru/projects/symengine.py/symengine/lib/symengine_wrapper.cpp:54321)
  File "symengine/lib/symengine_wrapper.pyx", line 2129, in symengine.lib.symengine_wrapper.DenseMatrixBase._applyfunc (/home/isuru/projects/symengine.py/symengine/lib/symengine_wrapper.cpp:54190)
  File "symengine/lib/symengine_wrapper.pyx", line 2359, in symengine.lib.symengine_wrapper.ImmutableDenseMatrix._set (/home/isuru/projects/symengine.py/symengine/lib/symengine_wrapper.cpp:61242)
TypeError: Cannot set values of <class 'symengine.lib.symengine_wrapper.ImmutableDenseMatrix'>

cc @ShikharJ

[ShikharJ]

The issue is that applyfunc and subs make use of _set which has been overloaded in ImmutableMatrix from its original implementation in DenseMatrixBase. I wasn't sure if that function was intended for general use or not, and hence I had overloaded the function. I'll remove that and add some tests.

[isuruf]

Also, row_join and col_join returns a MutableMatrix