Osgb

- Replaced OSTN02 data set with OSTN15, the new OSGB standard

- Moved OSTN data to external files

- Removed function to inspect OSTN data shifts

- Updated pod in OSGB.pm and OSGB/Background.pod

- Drop support for Perl 5.08 - if you must have 5.08 see the note in OSGB.pm

- Go faster stripes on main conversion routines

- Replaced three of the example scripts with a new single multifunction example script

- Fix small bug in Grid.pm: removed recent use of "//=", so that the module still works with Perl 5.8

- Minor refactoring to obey more of perl-critic and perl-tidy

- Fixed version numbers and date marks

- Put an ':all' tag back in OSGB.pm

- Restored ability to parse a grid reference consisting of a single sheet name

- Corrected pod in `build/make_maps_code`

- Minor updates to documentation

- More rigorous accuracy test in t/06osdata.t

- Added small bench mark test

- Fix bug in OSGB.pm with "unpack" under Perl V5.8

- Minor documentation corrections

- Clearer explanation of changes since V2.10 in README and Background.pod

- Changed examples/plot_maps to use proper OSTN02 polygons instead of just marking points

- Improved decimal degrees to degrees, minutes, seconds function in examples/bng_to_ll

- Fixed examples/plot_maps.pl so that it makes a better attempt to show more
than one map series on the same plot

- Minor corrections and additions to documentation

- Moved build utilities to new 'build' directory + improved documentation for them

- Packed OSTN02 data more densely so that OSGB.pm is smaller

- Corrected data points at eastern boundary of OSTN02 area