The local basis is passed into the local operator kernels as a const
reference.  That means, its lvalue is not a constant expression.  Would we
pass it by value, then it's lvalue would be a constant expression (through
it's rvalue still would not).

Calling a static function of a class throuh an object with the syntax
```c++
  lb.size()
```
will evaluate the lvalue of the object.  Since that lvalue is not constant
when the object was passed by reference, the call cannot be a constant
expression.

Clang (rightly) complained about this, so write the call as
```c++
LocalBases::size()
```
where a constant expression is required.

This uses our own Q1 local basis implementation that hands out std::array
results

Imported and adapted from dune-localfunctions q1localbasis

This is to appease clang 3.8

Closes: #4

Addresses: #6

Closes: #11

Addresses: #3

At the operator level, apply() is the equivalent of mv(), not umv().

Closes: #10

Addresses: #9

Addresses: #9

Closes: #2

Surprisingly, the reference vtu's still work...

Not available in C++14, see
https://zivgitlab.uni-muenster.de/jfahl_01/pacxx-projectseminar-2019/-/jobs/108746

Dof numbering assumes Q1/P1 anyway

We were assuming p1/Q1 layout anyway, so don't bother