Buffer for Explicit/SemiImplicit

[termoshtt]

Resolve #33

New

• ExplicitBuf
• SemiImplicitBuf

Implementations

• explicit module implements schemes for ExplicitBuf
• semi_implicit module does for SemiImplicitBuf, and revise Buffer setting

Rename

• WithBuffer -> BufferSpec since there is NoBuffer
• ModelSize -> ModelSpec to match BufferSpec

[termoshtt]

Bug: Accuracy of RK4 does not scale dt^4

[termoshtt]

Bug is in test code (´・ω・｀)