ops: eigen: revise overload constraints of norms

refs #450

Eigen overloads of norms:

function	input	remarks
`norm1` `norm2`	Eigen vector or rank-1 expression	requires underlying scalar type to be known integral of floating-point type

Unit tests (in tests/functional_small/ops/):

file name	test name	tested type
`ops_eigen_vector.cc`	`ops_eigen.vector_norm1` `ops_eigen.vector_norm2`	`Eigen::VectorXd`
`ops_eigen_diag.cc`	`ops_eigen.diag_norms`	`pressio::diag( Eigen::MatrixXd )`
`ops_eigen_span.cc`	`ops_eigen.span_norms`	`pressio::span( Eigen::VectorXd )`

Pressio / pressio