Typo in coefficient_common.i

[goodman17c]

This error occurs in the _ser build of PyMFEM within the coefficient_common.i file
In lines 67 and 69, there is a typo, where agrs is used rather than args resulting in python error when a sumcoefficient is used.

%pythonappend SumCoefficient::SumCoefficient %{
    if len(args) > 0 and isinstance(agrs[0], Coefficient):
          self.linkA = args[0]
    if len(args) > 1 and isinstance(agrs[1], Coefficient):
          self.linkB = args[0]

[goodman17c]

#182 I submitted a PR to resolve this bug.

[sshiraiwa]

Thank you so much!