Add support for Neuman and Robin boundary conditions about gridap.jl

[fverdugo]

In order to do this work, we need the new functionality that will be available after the ongoing re-factoring of CellValues (see issue #30).

In particular, we need that indexable cell values, remain indexable after operations with them. (Now, operations always return iterable CellValues) .

We also need to "reindex" indexable CellValues, e.g., to restrict a CellValue to a sub-set of its indices

EDIT: This has been done in PR #31

[fverdugo]

After commit e981f3c we can restrict CellFields and CellBasis objects to surfaces. However, some things need to be optimized and enhanced:

• We need to efficiently implement reindex for cell fields arising from applying kernels. Be carefull with objects resulting from applying the gradient. Done it commit 952359e
• Extend to non oriented faces For Neuman and Robin BCs this is not needed. It will be required to integrate in the skeleton.
• FullGridGraph not strictly needed for building a BoundaryGrid. Item moved to its own issue #44
• Make FEFunction indexable, and implement an efficient version of restrict for it. Done in commit ca77bb3

[fverdugo]

First example with Neumann BCs in commit 4dcd16f

We do not close the issue until we test Robin conditions and improve the implementation of reindex

[fverdugo]

First example with Robin BCs in commit 946054a

[fverdugo]

This work has finished with commit 952359e

The only missing part is moved to a separate issue (issue #44)

Closing issue