WIP: Corner frequency and fundamental frequency in PLRedNoise and PLDMNoise

[abhisrkckl]

See #1366

Fundamental frequency

The TNRedFLow parameter can control the fundamental frequency of the red noise. The relevant expression is

F_1 = 10**TNRedFLow / Tspan

where Tspan is the data span.
This definition is adopted to be consistent with tempo2.

The corresponding parameter for the DM noise is TNDMFLow. I have kept its semantics similar to TNRedFLow although it's not yet implemented in tempo2.

Corner frequency

When corner frequency is specified, the expression for the power spectrum is

As fc -> 0, this reduces to

The parameter TNRedCorner is defined in tempo2 as

f_c = TNRedCorner / Tspan

[codecov]

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@@            Coverage Diff             @@
##           master    #1762      +/-   ##
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- Coverage   69.43%   69.32%   -0.11%     
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  Files         107      106       -1     
  Lines       24925    24895      -30     
  Branches     4430     4432       +2     
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- Hits        17306    17258      -48     
- Misses       6510     6522      +12     
- Partials     1109     1115       +6     

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