Improve Fscan spec_* codes

Description

This MR improves the Fscan spec_* codes in two main ways:

• moves a commonly used function to fscanutils library so that the function can be used in different Fscan programs
• rewrite the spec_avg program so that it reads one SFT at a time, potentially making it possible to run a single instance of the program over an entire 2 kHz band over an entire month or more. The program would emit 1 file per subband for the normalized-averaged spectra, but only one file for the spectrogram (full band), and one file for SFT GPS timestamps and one file for human-readable date times for each SFT

Specifically, the following changes in outputs would occur with this MR:

File type	Before	After
GPS timestamp file	one	one
datetime timestamp file	one	one
Spectrogram file	one	one
Normalized, averaged spectra file	one	potentially multiple (if subband < f_max - f_min)

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

The only function changed is extract_one_sft() and the only use of this function is within Fscan codes.

Review Status

@ansel-neunzert, @karl-wette, or @david-keitel should review this once it is out of draft.

We should check with @keith-riles to ask if these changes would impact any online production of spectra. The only impact would be if Keith uses lalpulsar_spec_avg. I believe he does not, since it should be used by Fscan exclusively.