Merging the things

Further clean up DGDiscretizationWithBoundaries.{discr_from_dd,connection_from_dds}

See merge request inducer/grudge!17

Clean up the thicket of ifs in connection_from_dds

See merge request inducer/grudge!15

EM Maxwell, but cleaner

See merge request inducer/grudge!10

Rename grudge.Discretization to DGDiscretizationWithBoundaries, refactor…

See merge request inducer/grudge!14

Rename grudge.Discretization to DGDiscretizationWithBoundaries, refactor sub-discr/conn getter interfaces

Centralize decision on group factories (thanks @eshoag2 for the idea) (Fixes #6 on gitlab)

Closes #6

See merge request !13

Rename FRESTR symbols to something more self-explanatory (thanks @eshoag2)

See merge request !12

Basic fusion

See merge request !7