Initialise pmod structure in pspm_glm

[dominikbach]

Feature Description

When multiple onset files are combined in pspm_get_timing, then the consistency of the variable "names" is checked: this must have the same values across all sessions. However, associated "pmod" are not checked.
This can lead to the following edge case (see issue #191): a particular condition only occurs in session k. Only the condition file for that session defines an associated pmod. This leads to an error in pspm_glm if k > 1.
The pmod structure should be initialised based on the pmods in all sessions.
For more details see #191.