Tidy3d

- A few small fixes.

- Fixed nonlinear constraint in dispersive material fitting tool.
- Fixed potential issue when a monitor stores neither `'E'` nor `'H'`.
- Fixed some backwards compatibility issues introduced in 21.3.1.5.

- Frequency monitors can now optionally store the complex permittivity at the same locations where
the E-fields are recorded, at the monitor frequencies.
- Frequency monitors now also have an `'interpolate'` keyword, which defaults to `True` and
reproduces the behavior of previous versions. If set to `False`, however, the raw fields
evaluated at their corresponding Yee grid locations are returned, instead of the fields interpolated
to the Yee cell centers. This also affects the returned permittivity, if requested.
- Reorganized internal source time dependence handling, enabling more complicated functionality
in the future, like custom source time.
- Total field in the simulation now sampled at the time step of the peak of the source time dependence,
for better estimation of the shutoff factor.
- A number of bug fixes, especially in the new plotting introduced in 21.3.1.4.

- Reorganized plotting:
- Speeding up structure visualizations.
- Structures now shown based on primitive definitions rather than grid discretization. This
then shows the physical structures but not what the simulation "sees". Will add an option to
display the grid lines in next version.
- Bumped down matplotlib version requirement to 3.2 and python version requirement to 3.6.
- Improved handling of PEC interfaces.- Reorganized and extended internal logging.
- Added ``tidy3d.__version__``.
- A number of fixes to the example notebooks and the colab integration.

- Bumping back python version requirement from 3.8 to 3.7.

- Hotfix to an internal bug in some simulation runs.