Bugfix/issue 530 no model parameters

### Summary

Fixes issue #530 so that, instead of crashing, the program outputs an error message stating that algorithm=fixed_param must be used if the model has no parameters. Also adds a test for non-zero length to ps_point::fast_vector_copy and ps_point::fast_matrix_copy.

### Intended Effect

Get rid of a mysterious crash that occurs if you have just a generated quantities block, which one might do when experimenting with Stan to understand what various functions do.

### How to Verify

Some new cases were added to fixed_param_sampler_test.cpp.

### Side Effects

None

### Documentation

Manual modified to mention that algorithm=fixed_param is mandatory when the parameters block is empty.