Add support for multiple FOV types in SpecularCoverage class

[ryanketzner]

Currently, the SpecularCoverage class only works with CircularFieldOfVIew, but it can easily be extended to any FOV like the other CoverageCalculator class. From coveragecalculator.py:

if isinstance(fov, CircularFieldOfView):
    deg2rad = math.pi / 180.0
    half_angle_rad = fov.diameter * 0.5 * deg2rad
    boresight_fov_gte = gte.Vector3d(fov.boresight)

    # First, define the boresight vector and cone angle in the FOV frame as a constant
    # source.
    boresight_fov_source = kcl.ConstantSourceVector3d(boresight_fov_gte)
    angle_source = kcl.ConstantSourced(half_angle_rad)

    # Define a source which transforms the boresight from the FOV frame to the targe frame.
    # This is a Variable object so it must be added to the variables list to be updated.
    boresight_target_source = kcl.TransformedVector3dSource(
        boresight_fov_source, fov_to_target_source, buff_size
    )

    # Define a source which builds a cone object using the FOV position, boresight
    # unit vector, and cone half-angle.
    # This is a Variable object so it must be added to the variables list to be updated.
    fov_source = kcl.PosDirConeSource(
        pos_fov_target_source,
        boresight_target_source,
        angle_source,
        buff_size,
    )

    # Define a Viewer object for the cone shape. Viewer objects are used to make the shape
    # compatible with constructive solid geometry (CSG) operations.
    fov_viewer = kcl.ViewerCone3d(fov_source)

    # Add variables to list
    variables.append(boresight_target_source)
    variables.append(fov_source)
else:
    raise ValueError("Unsupported field of view type.")