A performance issue

There may be performance issues in FerriteGmsh, as importing such a small-scale grid actually took 2 minutes.
```
using Ferrite, FerriteGmsh
@time grid = togrid("periodic-rve.msh")
```
Output:
```
Info : Reading 'periodic-rve.msh'...
Info : 38 entities
Info : 6097 nodes
Info : 12192 elements
Info : Done reading 'periodic-rve.msh'
119.920582 seconds (132.54 k allocations: 8.574 MiB)
Grid{2, Triangle, Float64} with 11904 Triangle cells and 6097 nodes
```