Faster parameter priors; no float128s in Pulsar; dict-like interface for PTA and SignalCollection

[vallis]

...for SignalCollection access in PTA and Signal access in SignalCollection. So one can do

>>> pta
<Enterprise PTA object: B1855+09, B1937+21, B1953+29>
>>> pta[0]
<Enterprise SignalCollection object B1855+09: linear_timing_model, efac, equad, ecorr_sherman-morrison, red_noise, gw>
>>> pta['B1855+09']
<Enterprise SignalCollection object B1855+09: linear_timing_model, efac, equad, ecorr_sherman-morrison, red_noise, gw>
>>> pta['B1855+09'][0]
<enterprise.signals.gp_signals.TimingModel.<locals>.TimingModel at 0x7fb4a1ef6d90>
>>> pta['B1855+09']['linear_timing_model']
<enterprise.signals.gp_signals.TimingModel.<locals>.TimingModel at 0x7fb4a1ef6d90>

Also, renamed FourierBasisCommonGP.signal_name as 'common red noise'.

[codecov[bot]]

Codecov Report

Merging #302 (172c814) into master (e2e0505) will increase coverage by 0.16%. The diff coverage is 91.83%.

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
+ Coverage   87.14%   87.31%   +0.16%     
==========================================
  Files          13       13              
  Lines        2870     2908      +38     
==========================================
+ Hits         2501     2539      +38     
  Misses        369      369              

Impacted Files	Coverage Δ
enterprise/signals/signal_base.py	90.31% <88.23%> (+0.03%)	:arrow_up:
enterprise/pulsar.py	92.36% <100.00%> (ø)
enterprise/signals/gp_signals.py	88.07% <100.00%> (+0.07%)	:arrow_up:
enterprise/signals/parameter.py	78.62% <100.00%> (+1.30%)	:arrow_up:

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[vallis]

Now including also:

• faster non-scipy versions of parameter.Uniform and parameter.Normal priors and samplers (responding to #307);
• a Pulsar tweak that downgrades psr.dm and psr.dmx to have float64 rather than float128 values, so that Pulsar pickles can be read on arm64 platforms.