Pyuvdata

Added
- `keep_all_metadata` keyword for optionally discarding unused metadata when performing a select operation.

Changed
- Extends `run_acceptability_check` for UVH5 metadata in `check_header` function.

Fixed
- Antenna numbering bug in redundancy methods. It wasn't using the correct antenna numbers to make baseline indices.
- Redundancy code returns one group if all baselines are redundant. Previously returned each baseline as a separate group
- Redundancy code finds unique baselines along baseline_array without assuming Nblts = Nbls \* Ntimes. Previously assumed Nblts = Nbls \* Ntimes and attempted to slice array.
- "inflate_by_redundancy" method errored when phase_type == phased, due to _set_u_positive using phased uvw coordinates. It now uses ENU frame uvw coordinates.

Added
- Methods on UVData objects to compress/inflate data by redundant baselines.
- Convenience functions on UVData for finding redundant baselines (calling the corresponding utils functions)
- memo describing the UVH5 format
- read/write support for uvh5 files with integer datatypes for visibilities
- Option to only do the select on the metadata. This is useful for partially defined objects as in pyuvsim setup or after reading only the metadata from a file.
- support for python3.7

Changed
- UVdata.get_ENU_antpos() now defaults to using the telescope_location as the center rather than the median antenna position.
- UVBeam.efield_to_pstokes() no longer restricted to healpix coordinates
- latitude and longitude in uvh5 files are written in degrees instead of radians.
- Fixes a bug in redundancy methods for when there are no redundant baselines.

Fixed
- `_key2inds` now properly reorders polarization axis for conjugated visibilities. This also effects the `get_data` function.
- long strings are saved correctly in miriad files from python3

Added
- option to save splines for reuse in UVBeam.interp function

Changed
- improve string handling for uvh5 files
- changed top-level import structure to exclude file-specific class (e.g. `UVFITS`, `CALFITS`) and base classes (`UVBase`, `UVParameter`) and to not import utility functions into the top-level namespace

Deprecated
- Support for UVData objects without antenna_positions. Antenna positions will be required in a future version.

Added
- Utility functions to identify redundant baselines (either by baseline or antenna pair)
- Reading FHD layout files to get antenna positions
- Polarization dict constants and parsing functions in utils.py for mapping between polarization strings and numbers

Changed
- LST array is now optional in uvh5 files
- polarization string capitalization was unified throughout: use lower case except for Stokes parameters
- integration_time is now a vector of length NBlts instead of a scalar

Deprecated
- Support for FHD inputs without layout files (containing antenna positions).

Fixed
- flags are always returned as a boolean array from `UVData.get_flags`
- integration_time, uvw_array and lst_array are now always checked for consistency when adding UVData objects
- consistency checks on baseline lengths now uses the uvw_array tolerances