ops: eigen: revise `dot` overload constraints

refs #524

Eigen dot overloads:

function	input	remarks
`void dot(a, b, scalar)` `scalar dot(a, b)`	Eigen vectors or rank-1 expressions	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_dot`	`Eigen::VectorXd`
`ops_eigen_diag.cc`	`ops_eigen.diag_dot_diag` `ops_eigen.diag_dot_vector`	`pressio::diag( Eigen::MatrixXd )`
`ops_eigen_span.cc`	`ops_eigen.span_dot`	`pressio::span( Eigen::VectorXd )`

Pressio / pressio