Stan signature expression tests should support complex numbers

[WardBrian]

Some math tests are triggered off the output of signatures from stanc. These were never updated to include complex numbers as a scalar, and particularly with complex_matrix etc coming we should add this.

See:

math/test/sig_utils.py

Lines 15 to 27 in 01dd2e7

	arg_types = {
	"int": "int",
	"array[] int": "std::vector<int>",
	"array[,] int": "std::vector<std::vector<int>>",
	"real": "SCALAR",
	"array[] real": "std::vector<SCALAR>",
	"array[,] real": "std::vector<std::vector<SCALAR>>",
	"vector": "Eigen::Matrix<SCALAR, Eigen::Dynamic, 1>",
	"array[] vector": "std::vector<Eigen::Matrix<SCALAR, Eigen::Dynamic, 1>>",
	"row_vector": "Eigen::Matrix<SCALAR, 1, Eigen::Dynamic>",
	"array[] row_vector": "std::vector<Eigen::Matrix<SCALAR, 1, Eigen::Dynamic>>",
	"matrix": "Eigen::Matrix<SCALAR, Eigen::Dynamic, Eigen::Dynamic>",
	}

This will need to add mappings for complex (std::complex<SCALAR>) and similar for the container types

[WardBrian]

@rok-cesnovar would you be able to look into this?

[rok-cesnovar]

For sure.

These were never updated because we only needed expressions tests for Eigen types and there was no support for eigen complex types yet.

[WardBrian]

Great, thanks! This will also make the adding of more math signatures for the complex types in stanc much easier in the future

[SteveBronder]

@rok-cesnovar did you check out #2434 for this? Might be easy to override that and produce complex types instead of var mat types

[rok-cesnovar]

Thanks. Havent yet, currently focused mostly on getting #2670 to run so we get the patch release out.

[WardBrian]

No real rush on this, definitely after the patch is great