Pylas

- Changed the x, y, z offsets are not longer changed when new x,y or
are set

- Added `supported_version` function which returns the LAS version
supported by pylas
- Added a new `PointFormat` class
- Added a `merge_las` function
- Added `mins` & `maxs` properties to the header, they
provide access to the x, y, z mins and maxs as numpy arrays

- Fixed initialize the `header.date` to `date.today()`

- Updated lazperf to handle lazperf exrabytes

- Changed all pylas specific exception now inhereits the `PylasError`
exception class

- Fixed extra dimension bug where the extra field was added to
the LasData attrbute but not the the points array

- Initial version