reduce the number of pattern hash collisions in database construction

[smroid]

The default value of 'pattern_max_error' passed to generate_database() results in a large number of pattern hash collisions. This is because the default value 0.005 results in a 'pattern_bins' value of 50; this in turn limits the space of possible pattern hashes and so patterns with similar edge ratios are assigned identical quantized pattern hash values. A smaller 'pattern_max_error' value (e.g. 0.002) would raise 'pattern_bins' to 125, increasing the space of possible pattern hashes and thus decrease the rate of pattern hash collisions.

https://github.com/esa/tetra3/pull/25 fixed a bug that caused databases built with smaller 'pattern_max_error' values to fail to solve.

Remaining work:

• Add logic to measure and report the different kinds of collision that occur when generating a database. https://github.com/esa/tetra3/pull/26 is in review for this; this PR also refines terminology around the different kinds of hash collision inherent in the database design.
• Allow a different 'max_error' value to be used at solve time. This will allow a "precise" database to be used with "imprecise" solve-time images, e.g. when accurate FOV and/or distortion information is not yet available.

[smroid]

Note that https://github.com/esa/tetra3/compare/cc1ce4d77d81e4188d463c91958ae0d12ebd5dea...a91fa38e8d208b198a3dfde961671bd81db3ee16 corrects a bug that caused small 'pattern_max_error' values to be problematic at large FOVs.

[smroid]

Closing here, will continue work at smroid/cedar-solve.