Use precomputed PSR file for point sources if provided

[israelmcmc]

By Point Source Response (PSR) we refer to the expected excess on the data space (Em, CDS) for a source located at a given point in the sky. We keep the photon energy "Ei" to compute the excess for various spectra.

The PSR can be computed on the fly given a FullDetectorResponse. When we do an analysis in Galactic coordinates, we need to integrate over the various SC orientations, which is slow. For a point source this is still manageable, and it is indeed computed on the fly, but for extended or all-sky sources we had to pre-computed for a given time range (e.g. the 3 months of DC2) and for multiple directions, and provide it to the user. This is passed to COSIlike as:

cosipy/cosipy/threeml/COSILike.py

Lines 62 to 63 in aea580b

	precomputed_psr_file : str, optional
	Full path to precomputed point source response in Galactic coordinates

If provided, it is used for extended sources in COSILike, but for point sources, we are still creating it on the fly:

cosipy/cosipy/threeml/COSILike.py

Lines 199 to 201 in aea580b

	elif self._coordsys == 'galactic':
	scatt_map = self._get_scatt_map(coord)
	self._psr[name] = self._dr.get_point_source_response(coord=coord, scatt_map=scatt_map)

While not prohibitively slow, it would be better to use the pre-computed PSR for point sources if the user provides it.

See also PR #284, and issues #129 and #267.

When this issue is solved, we can remove this comment in the code:

cosipy/cosipy/threeml/COSILike.py

Lines 117 to 120 in aea580b

	# Note: this still needs to be implemented in a
	# consistent way for point srcs and extended srcs.
	self.precomputed_psr_file = precomputed_psr_file
	if self.precomputed_psr_file != None: