Closer integration with Thetis

[jwallwork23]

Whilst adapt_utils uses the Thetis solver structure and inherits much of the machinery, it can easily get out of date. For example, the recent change in SIPG formulation means that they are not synchronised. To fix this, we should tie adapt_utils more closely to Thetis. For example, Options could be a subclass of Thetis' 2d options class.