Replacing GISMO_UNUSED with EIGEN_UNUSED_VARIABLE in gsEigen

[dominik-mokris-mtu]

Changes

FIXED: I have replaced GISMO_UNUSED with EIGEN_UNUSED_VARIABLE in external/gsEigen.

TL;DR

With the GISMO_UNUSED I cannot use solution 5 from #566 anymore.

Longer explanation

I am compiling an application that depends both G+Smo and another library that also uses Eigen. In order to prevent issues from having two copies of Eigen, the following solution has worked great in the past:

1. install G+Smo with make install,
2. compile the other library -I-ing the content of the gsEigen that G+Smo made in step 1,
3. compile my application linking to the results of steps 1 and 2.

Step 2. currently fails due to GISMO_UNUSED not being defined. After replacing it with EIGEN_UNUSED_VARIABLE, step 2 works without problems. In my view, the content of the external/gsEigen should generally not depend on anything G+Smo specific.

The checklist

• Have you added an explanation of what your changes do and why
  you'd like us to include them? Yes, see above.
• Have you documented any new codes using Doxygen comments? No, I don't think it is necessary.
• Have you written new tests or examples for your changes? No, I don't think it is necessary.

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