Skip geometry validation in simulation subsection and mode solver reduction

[weiliangjin2021]

This is part of efforts in speeding up mode solver when a geometry group contains large number of geometries. (Last piece in resolving https://github.com/flexcompute/tidy3d-core/issues/692)

• Add options in updated_copy to skip valiation.
• The geometry is validated when the simulation object is initialized, and they are not modified in subsection. Thus we can skip validators that are directly associated with geometry.
• For validators of other simulation fields associated with geometry, only the bounding box of the geometry matters. So to validate other simulation fields, we use an auxiliary simulation object whose geometry is replaced by its bounding box.

[weiliangjin2021]

Total mode computation time in a test:

• develop branch: 1000s
• pre/2.8 with recent parallel computing changes: 600s
• pre/2.8 + this PR: 200s

[tylerflex]

HI Weiliang, a few higher level comments about the subsection options.

The new validate option looks good to me!

[momchil-flex]

I'm pretty happy with this, I mean it definitely feels somewhat hacky but also it achieves the performance boost we want while still doing just about as little of validation skipping as possible.

Agree with @tylerflex 's comments otherwise.