Create simulated data sets, FITS file following the header ICD

[joshwalawender]

Generate a set of simulated data files (FITS files) which can be used to test our data model. These simulated data should cover a range of possible formats and should include formats which represent raw data from various instruments and formats which might exist only in intermediate processing steps.

One goal is to cover as many possibilities as possible to test that the data model (and eventually the primitives which act on that data model) correctly handle all aspects of the data including the array data itself, variance information, mask information, header information, and table data.

Types of input data

• Single data extension with simple header.
• Multi-extension. One data HDU, one variance HDU, both with headers.
• Multi-extension. One data HDU, one mask HDU, both with headers.
• Multi-extension. One data HDU, one variance HDU, one mask HDU, all with headers.
• Multi-extension. One header HDU, two data HDUs with headers.
• Multi-extension. One header HDU, two data HDUs, two mask HDUs, all with headers.
• Multi-extension. One header HDU, two data HDUs, two variance HDUs, all with headers.
• Multi-extension. One header HDU, two data HDUs, two mask HDUs, two variance HDUs, all with headers.
• Multi-extension. One data HDU, one variance HDU, one table HDU, all with headers.
• Multi-extension. One data HDU, one mask HDU, one table HDU, all with headers.
• Multi-extension. One data HDU, one variance HDU, one mask HDU, one table HDU, all with headers.

We should confirm that basic mathematical operators (addition, subtraction, multiplication, etc.) implemented in simple primitives propagate each type of data (pixel data, header, mask, variance, table) properly.

Examples:

• Addition of a constant should change data, but not mask or variance.
• Multiplication by a constant should change data and variance, but not mask.
• Addition of two images should propagate variance correctly if both input images have variance and should handle the case where only one input image has variance.
• etc.

Note that handling of mask and variance is embedded in astropy.nddata.NDData and astropy.nddata.CCDData if we choose to use those.