clean up TODO items

[bob-carpenter]

Summary:

The merge trailed a bunch of todo items:

stan/src/stan/command/stanc_helper.hpp:  // TODO(martincerny) help for standalone function compilation
stan/src/stan/command/stanc_helper.hpp:        // TODO(martincerny) Check that the -namespace flag is not set
stan/src/stan/command/stanc_helper.hpp:        // TODO(martincerny) Allow multiple namespaces
stan/src/stan/command/stanc_helper.hpp:        // TODO(martincerny) Check that the -name flag is not set
stan/src/stan/lang/compile_functions.hpp:      // TODO(martincerny) the model_name below probably should not be a
stan/src/stan/lang/compile_functions.hpp:      // TODO(martincerny) raise error or warning when other blocks than
stan/src/stan/lang/generator/generate_standalone_functions.hpp:      // TODO(martincerny) try to reduce the includes that are n

Also a bunch from me that have never been cleared up:

stan/src/stan/command/stanc_helper.hpp:          // TODO(carpenter): shouldn't this be .hpp without a main()?
stan/src/stan/lang/ast/sigs/function_signatures_def.hpp:      // TODO(carpenter): for threaded autodiff, requires double-check lock
stan/src/stan/lang/generator/write_array_visgen.hpp:      // TODO(carpenter): reuse cut-and-pasted from other lub reader case
stan/src/stan/lang/generator.hpp:// TODO(carpenter): move into AST
stan/src/stan/lang/generator.hpp:// TODO(carpenter): move into general utilities
stan/src/stan/lang/grammars/semantic_actions_def.cpp:    // TODO(carpenter): seems redundant; see if it can be removed

• [ ] fix the easy ones
• [ ] turn the rest into issues or just delete

Description:

In general, we should be using TODO only as a last resort.

Current Version:

v2.15.0