Adding unit tests for spatial functions.

[cbernalz]

Goal

We want to create tests for all new spatial functions added in the spatial-main branch.

Requirements

• [x] Add test for construct_spatial_rt.stan.
• [x] Add test for spatial_deviation_noise_matrix_rng.stan.
• [x] Add test for spatial_rt_process_rng.stan.
• [x] Add test for construct_aux_rt.stan.
• [x] Add test for state_deviation_noise_vec_aux_rng.stan.
• [x] Add test for aux_site_process_rng.stan.
• [x] Add test for equivalence of exponential_decay_corr_func.stan and exponential_decay_corr_func_r.R.
• [x] Add test for equivalence of independence_corr_func.stan and independence_corr_func.R.
• [x] Add test for equivalence of spatial_rt_process_rng.stan and spatial_rt_process.R.