NPI-3668 SP3 header stated values vs content checks, and ORB_TYPE update on performing Helmert transformation

[treefern]

This PR is intended to add a bit more robustness to our generated SP3 headers.

This adds:

• Checks that the list of SVs the header claims the SP3 data contains, match the values in the DataFrame itself (which will become the content of the SP3 file written out).
• Logic to set the ORB_TYPE field of the SP3 header to HLM, when a Helmert transformation is performed on the SP3 DataFrame. This is done to keep the header metadata used to write out headers, more consistent with changes made to the data itself.
• Some additional unit test assertions relating to header SV count and SV list length, and merging of SV accuracy codes (lowest common denominator, in effect).
• Various clarifying comments and variable name changes