[ENHANCEMENT] Support scheduling of calibration (dark, bias, flat frames) explicitly

[WWGolay]

Is your feature request related to a problem? Please describe.
Currently, users may select the shutter status via the shutter_state keyword in a .sch file or as a key + value pair in an astroplan ObservingBlock configuration dictionary. However, astroplan ObservingBlocks are required to include an RA and Dec for the astroplan PriorityScheduler to be able to schedule the block. Since darks and biases are necessarily spatially and temporally agnostic, this poses a problem for users who do not know a priori what times will be available for calibration frames during the night.

Additionally, users cannot schedule daytime calibration images since the TelrunOperator prevents observations from beginning before sunset.

Describe the solution you'd like

• schedtel needs to be modified to accommodate calibration frames that can occur at any time.
• TelrunOperator should be able to handle ObservingBlocks that do not include pointing information/other normally critical keywords in the configuration and then recognize it as a calibration image.
• TelrunOperator needs modification to accept daytime calibration observations.
• Horizon and CoverCalibrator flats have particular constraints, e.g., pointing and timing, that should be considered in both schedtel and the TelrunOperator.

Describe alternatives you've considered
A "daytime" schedule may be considered with different syntax that the TelrunOperator handles differently.

Additional context
The automation of calibration images is geared towards the larger goal of a looping script similar to syncfiles that will periodically generate timestamped updated calibrations.