Add MockPulsar class, for easy use of Mock data with Enterprise

[vhaasteren]

This adds a very rudimentary MockPulsar class: a class that can be easily instantiated without a timing package (PINT/Tempo2). All Enterprise models can be run on this type of pulsar object, but it can be instantiated from mock data very quickly. This is useful for anyone who wants to use Enterprise on fake data. Previously, one had to rely on fake par/tim files, created through libstempo.toasim or otherwise.

Note: this is coded up as part of Enterprise, and not Enterprise_extensions, because the pulsar class is essential for usage of Enterprise. Allowing Enterprise to be useful without a timing package was a big plus, but with libstempo/PINT optional I guess that's less of an advantage now

Todo:

• Create unit tests
• Add Astrometry parameters, aside from only Spindown

Additional todo, perhaps not part of this PR:

• Add helper functions to enterprise_extensions so this class can be instantiated easily
• Resolve unit disparity of PMRA / PMDEC between Tempo2 and PINT
• Add documentation for Tempo2Pulsar and PintPulsar

As in the test suite, a typical way to add this pulsar is:

toas = np.linspace(53000.0, 58000.0, 4005)
flags = {"f": np.array(["nosystem"] * 4005), "fe": np.array(["nofrontend"] * 4005)}
decj, raj = (0.16848694562363042, 4.9533700839400492)
# Can also define elong/elat

psr = MockPulsar(
    obs_times_mjd=toas,
    raj=raj,
    decj=decj,
    ssbfreqs=1440.0 * np.ones_like(toas),
    residuals=np.zeros_like(toas),
    toaerrs=1e-6 * np.ones_like(toas),
    sort=True,
    flags=flags,
    telescope="GBT",
    spindown_order=2,
    inc_astrometry=True,
)

This pulsar now has psr.name == 'J1855+0939', and the design matrix contains astrometry and spin models (so 8 parameters). Residuals can be set with:

psr.set_residuals(residuals)

This pulsar is extremely fast to make, and can be combined with most Enterprise functionality. Astrometry parameters depend on Astropy (optional dependency).

[codecov]

Codecov Report

Attention: Patch coverage is 98.62069% with 2 lines in your changes missing coverage. Please review.

Project coverage is 73.31%. Comparing base (e82c676) to head (49180cc).
Report is 17 commits behind head on dev.

Files with missing lines	Patch %	Lines
enterprise/pulsar.py	94.44%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #361      +/-   ##
==========================================
+ Coverage   71.53%   73.31%   +1.77%     
==========================================
  Files          13       13              
  Lines        3246     3391     +145     
==========================================
+ Hits         2322     2486     +164     
+ Misses        924      905      -19     

Files with missing lines	Coverage Δ
enterprise/signals/utils.py	85.29% <100.00%> (+3.18%)	⬆️
enterprise/pulsar.py	36.61% <94.44%> (+8.04%)	⬆️

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