v2.0.0

This is a major refactor of the code base to improve the overall structure and documentation.
Tide model read functions have been renamed and placed in the io directory
Additional capabilities for reading and writing files have been added
Interpolation routines have been rewritten into a single module
For now, the code should be the backwards compatible with deprecation warnings

• refactor: single implicit import of pyTMD tools (#130)
• refactor: reorganization of tide model readers under io (#132)
• refactor: placed interpolation routines into new module (#141)
• refactor: move model class to io
• feat: add constants class for ellipsoidal and gravitational parameters (#135)
• feat: functions to read and interpolate from all constituents (#137)
• feat: new functions to output ATLAS, FES and GOT netCDF4 files (#139)
• feat: output variables for some standard epochs used within tide programs
• feat: update forecast notebook with dynamic plotting
• fix: copy input coordinates within read functions so they are not transformed
• docs: update documentation for new structure
• docs: standardized citation format throughout docstrings
• docs: add release notes for all prior public releases
• docs: add new pyTMD logo
• test: read header from OPT test file and compare more variables
• test: add tests for io methods