Solve beam on Initialization

Add ability (automatically?) to solve a beam on the same line it is initialized. 

For example, the following is not currently possible, but would be convenient.

```
beam = Beam(...).solve()
# further analysis on beam, ie
beam.plot()
...
```

Maybe even solve the beam automatically when it is initialized so that all you need is 

```
beam = Beam(...)

# analyze beam
```
