use field_type( 1 ) instead of 1.0

This fixes FS#1317 for me (use invert for 2x2 matrix with
std::complex< long double >).
The problem is that 1.0 is a double, which can be converted implicitly
to a long double which, in turn, could be converted implicitly into a
std::complex< long double >. However, the compiler does not use two
implicit type conversions, so double cannot be divided by a
std::complex< long double >.

dune/common/densematrix.hh

@@ -867,7 +867,7 @@ namespace Dune
       if (fvmeta::absreal((*this)[0][0])<FMatrixPrecision<>::absolute_limit())
         DUNE_THROW(FMatrixError,"matrix is singular");
 #endif
-      (*this)[0][0] = 1.0/(*this)[0][0];
+      (*this)[0][0] = field_type( 1 ) / (*this)[0][0];
 
     }
     else if (rows()==2) {
@@ -877,7 +877,7 @@ namespace Dune
       if (fvmeta::absreal(detinv)<FMatrixPrecision<>::absolute_limit())
         DUNE_THROW(FMatrixError,"matrix is singular");
 #endif
-      detinv = 1.0/detinv;
+      detinv = field_type( 1 ) / detinv;
 
       field_type temp=(*this)[0][0];
       (*this)[0][0] =  (*this)[1][1]*detinv;