XLALParseSFTFilenameIntoSpec(): parse old style SFT names

Description

This MR relaxes the SFT filename parsing function XLALParseSFTFilenameIntoSpec() to be backward compatible with 2 common SFT filename usages/conventions from before the v3 SFT spec:

• Multiple underscores in the miscellaneous description field, e.g. L-1_L1_1800SFT_old_style_SFT_name-1238239230-1800.sft.
• The unofficial splitSFTs narrow-band SFT filename convention, where the NBF... frequency-band descriptor appeared before the miscellaneous description field, e.g. L-1_L1_1800SFT_NBF0010Hz0W0008Hz0_old_style_SFT_name-1238239230-1800.sft.

The SFTFilenameSpec filled by XLALParseSFTFilenameIntoSpec() will always be a valid SFT filename according to the v3 spec. For example, in the above filenames, the SFTFilenameSpec.privMisc field would be set to oldstyleSFTname, i.e. with no underscores per the spec. As a result XLALBuildSFTFilenameFromSpec() will always generate a v3-compliant name.

Closes #695 (closed)

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

Review Status

cc @david-keitel