Esig

- Added Python interface in the package root to computation backends
- Added iisignature backend to allow computation of signatures using the iisignature package
- Added support for computing depth 1 signatures and logsignatures
- Added new unit tests.
- Made some changes to the build script
- Added Python 3.9 build target

- Fixed message displayed for Python versions below 3.8 when importing esig

- Added recombine function