Be strict about constant expressions

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.

src/nitschenonlinearpoissonfem.hh

@@ -71,7 +71,7 @@ namespace PPS {
       // geometry of inside cell
       auto geo_inside = isect.inside().geometry();
 
-      std::array<typename LocalBasis::Jacobian, lb.size()> gradphi;
+      std::array<typename LocalBasis::Jacobian, LocalBasis::size()> gradphi;
 
       // loop over quadrature points
       for (const auto& ip : PPS::quadratureRule(geo,order))
@@ -141,7 +141,7 @@ namespace PPS {
       // geometry of inside cell
       auto geo_inside = isect.inside().geometry();
 
-      std::array<typename LocalBasis::Jacobian, lb.size()> gradphi;
+      std::array<typename LocalBasis::Jacobian, LocalBasis::size()> gradphi;
 
       // loop over quadrature points
       for (const auto& ip : PPS::quadratureRule(geo,order))

src/nonlinearpoissonfem.hh

@@ -124,7 +124,7 @@ namespace PPS {
       auto geo = elem.geometry();
       const int order = incrementorder() + 2*lb.order();
 
-      std::array<typename LocalBasis::Jacobian, lb.size()> gradphi;
+      std::array<typename LocalBasis::Jacobian, LocalBasis::size()> gradphi;
 
       // loop over quadrature points
       for (const auto& ip : PPS::quadratureRule(geo, order))
@@ -176,7 +176,7 @@ namespace PPS {
       auto geo = elem.geometry();
       const int order = incrementorder() + 2*lb.order();
 
-      std::array<typename LocalBasis::Jacobian, lb.size()> gradphi;
+      std::array<typename LocalBasis::Jacobian, LocalBasis::size()> gradphi;
 
       // loop over quadrature points
       for (const auto& ip : PPS::quadratureRule(geo, order))