Add necessary test functions

[teddychao]

Feature Description

Many functions that have been implemented in the current version of PsPM do not have a corresponding test function. This issue monitors all the functions which requests test functions. The list of source functions which need test functions can be found here.

[teddychao]

@dominikbach Please check the list of functions waiting for adding corresponding test functions here. Thanks.

[dominikbach]

In my view, the most important ones are the core functions: pspmbf... pspm_con1 pspm_compute_visual_angle pspm_convert... pspm_downsample pspmget... pspm_merge pspm_time2index pspm_write_channel