Reconsider stimulus_presentations with gray screen and movie

**Is your feature request related to a problem? Please describe.**
stimulus_presentations now include gray screen and movie, but while implementing it there has been changes in other parts of the stimulus_presentations, such as 'omitted' and 'image_name'. Most of these were filled with NaN, which breaks downstream analysis codes such as GLM

**Describe the solution you'd like**
I think there are 2 options:
1. Having a separate dataFrame for including the full time series
2. Changing NaN to other values (this requires more discussion on which analysis codes are going to be impacted).

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.